In the first Klingon episode, "Errand of Mercy", Kirk tells the Organians:
KIRK: Gentlemen, I have seen what the Klingons do to planets like yours. They are organised into vast slave labour camps. No freedoms whatsoever. Your goods will be confiscated. Hostages taken and killed, your leaders confined. You'd be far better off on a penal planet. Infinitely better off.
And Kor tells them:
KOR: Good honest hatred. Very refreshing. However, it makes no difference whether you welcome me or not. I am here and will stay. You are now subjects of the Klingon Empire. You'll find there are many rules and regulations. They will be posted. Violation of the smallest of them will be punished by death.
And:
KOR: Yes. I am. I shall need a representative from among you. Liaison between the forces of the occupation and the civil population. Smile and smile. I don't trust men who smile too much. You, Baroner, you're the man.
KIRK: Me? I don't want the job.
KOR: Have I asked whether or not you want it? We Klingons have a reputation for ruthlessness. You will find that it is deserved. Should one Klingon soldier be killed, a thousand Organians will die. I will have order. Is that clear?
AYELBORNE: Commander, I assure you our people want nothing but peace. We shall cause you no trouble.
And:
KOR: From this day on, no public assemblages of more than three people. All publications to be cleared through this office. Neighbourhood controls will be established, hostages selected. A somewhat lengthy list of crimes against the state.
Clearly when the Klingons conquer inhabited planets the populations are not exterminated - Kirk would have said so if that was the case - but made subjects of the Klingon Empire and treated badly and harshly.
Note that the Klingons aren't the least little bit surprised that the Organians look like Earth Humans. Why should they be, since the most common appearance for extraterrestrials in TOS is looking like Earth Humans, and the TOS Klingons themselves only look slightly exotic and alien. Thus it is probable that many alien subjects of the Klingon Empire who happen to look like Earth Humans are used as spies in the Federation.
In "Balance of Terror", the first episode with Romulans, Kirk's first log entry says:
Captain's Log, stardate 1709.2. Patrolling outposts guarding the neutral zone between planets Romulus and Remus and the rest of the galaxy, received emergency call from outpost 4. The U.S.S. Enterprise is moving to investigate and assist.
If the neutral zone is actually between planets Romulus and Remus and the rest of the galaxy it would have to be a concentric shell surrounding the solar system(s) containing the planets Romulus and Remus. An inner sphere would be the Romulan edge of the neutral zone and an outer sphere would be the Federation edge of the neutral zone.
But since the map doesn't show the neutral zone going all the way around the system(s) of Romulus and Remus there is no way of knowing if it does go all the way around, outside the edges of the map that is shown, or if it is only a partial barrier between the Romulan Star Empire and the Federation.
Later two Romulans talk like they have seen constant warfare despite the neutral zone between them and the Federation.
COMMANDER: Danger and I are old companions.
CENTURION: We've seen a hundred campaigns together, and still I do not understand you.
COMMANDER: I think you do. No need to tell you what happens when we reach home with proof of the Earthmen's weakness. And we will have proof. The Earth commander will follow. He must. When he attacks, we will destroy him. Our gift to the homeland, another war.
CENTURION: If we are the strong, isn't this the signal for war?
COMMANDER: Must it always be so? How many comrades have we lost in this way?
CENTURION: Our portion, Commander, is obedience.
COMMANDER: Obedience. Duty. Death and more death. Soon even enough for the Praetor's taste. Centurion, I find myself wishing for destruction before we can return. Worry not. Like you, I am too well-trained in my duty to permit it. Continue evasive manoeuvres. Now, back to the first course.
If the frequency of campaigns is between four campaigns per year and four years per campaign, the hundred campaigns should have lasted between 25 years and 400 years. The hundred campaigns could have been in one foreign or civil war or each could have been a separate war.
It is quite possible, though not certain, that each campaign resulted in the conquest of a space travelling species or even a federation of space travelling species.
After discovering that outposts have been attacked, Kirk orders:
KIRK: Open a channel to our nearest command base. Quarter hour reports on our position and status.
UHURA: Yes, sir.
Later:
STILES: We'll enter the Neutral Zone in less than an hour, sir. Assuming, of course, that we don't turn back.
SCOTT [OC]: We now have aboard debris from Outpost four, Captain.
KIRK: Bring it to the briefing room. Are you ready, gentlemen?
Minutes later in the briefing room:
KIRK: What's our position?
UHURA [OC]: Course unchanged, sir. Estimating treaty boundary in twenty one minutes.
KIRK: Are you continuing to broadcast tactical reports?
UHURA [OC]: Affirmative, Captain.
KIRK: And at this distance?
UHURA [OC]: Approximately three hours before receiving a reply to our first message.
So Uhura should have broadcast at least one tactical report after the first one since Kirk asked if she was continuing to.
If Uhura broadcast two messages it should be between fifteen and thirty minutes after the first report, if she broadcast three it should be between thirty and forty five minutes after the first report, etc. Uhura says it should be about three more hours until they get a reply to the first message, so the subspace radio round trip time from the neutral zone to the nearest command base should be at least 3.25 hours.
Later:
STILES: We'll enter the Neutral Zone in one minute, Captain.
Kirk opens fire on the Romulans and:
STILES: Twenty seconds to Neutral Zone, sir.
KIRK: Lieutenant Uhura, inform Command base, In my opinion, no option. On my responsibility, we are proceeding into the Neutral Zone. Steady as we go, Mister Sulu. Continue firing.
Soon after both ships try a waiting game at the edge of the neutral zone.
Captain's log, Stardate 1709.6. We are at the Neutral Zone. Have lost contact with the intruder. No reaction on our motion sensors but believe the Romulan vessel to be somewhere close by. With all engines and systems shut down, the Enterprise is also playing the silent waiting game in hope of regaining contact.
Captain's log, supplemental. Now motionless for nine hours, forty seven minutes.
Nine hours 47 minutes is approximately 9.783 hours. This should be at least six hours after Uhura estimated they would get an answer to their first message.
The final battle should be just a few minutes later. And shortly after that battle:
KIRK: How many men did we lose, Bones?
MCCOY: Only one. Tomlinson. The boy who was getting married this morning. His fianc�e is at the chapel.
(Kirk turns to go, and Rand enters)
RAND: We finally received an answer from Command base, sir. They say they'll support whatever decision you have to make.
According to my rough calculations the action between the first scene and this scene takes somewhere between 10.383 hours and one day or 24.000 hours. Rand didn't specify if this answer was in response to the first message or in response to the message that they were going to enter the neutral zone.
The round trip time for subspace radio signals between the neutral zone and the nearest command base is somewhere between 3.25 hours and 24 hours. I would guess that if the message at the end was a reply to the message they were about to enter the neutral zone, the round trip time would be about 10 hours. That would mean that Uhura sent her first message about 6.75 hours before the briefing room scene.
If the message at the end was a response to the first message Uhura sent, then it came about seven hours later than it should have, so the staff at the command base would have spent those seven hours dithering, panicking, and running around like chickens with their heads cut off, or else waiting for an answer from Starfleet Command on Earth. That means that the total round trip time for a subspace message should be less than 24 hours minus 7 hours, or less than 17 hours.
The subspace radio round trip time from the neutral zone to the nearest command base is 3.25 to 17.00 hours, so the one way subspace radio trip time from the neutral zone to the nearest command base is about 1.625 to 8.5 hours.
When the
Enterprise crosses a different part of the neutral zone into Romulan space in "The Enterprise Incident" Sub Commander Tal give them one hour to surrender or be destroyed.
KIRK: You understand that Starfleet Command has been advised of the situation?
TAL [on viewscreen]: The subspace message will take three weeks to reach Starfleet. The decision is yours, Captain. One hour.
If three weeks is between 17.5 days (or 420 hours) and 24.5 days (or 588 hours), the travel time from this part of the Romulan border is 49.411 to 361.846 times as long as the travel time from another part of the Romulan border to the nearest command base in "Balance of Terror".
The round trip time for radio messages from Earth to the Moon and back is about 3 seconds, which caused a noticeable time lag in conversations with astronauts on the Moon.
There is no time lag when Spock at planet Gideon has a conversation with Admiral Fitzgerald at Starfleet Command in "The Mark of Gideaon", when Kirk near planet Daran V has a conversation with Admiral Westervleit at Starfleet Command in "For the World is Hollow and I Have Touched the Sky", or when Kirk near Sherman's Planet in an area disputed between the Federation and the Klingons has a conversation with Admiral Fitzpatrick in "The Trouble with Tribbles".
There is no time lag" in "Amok Time" when Kirk has a conversation with Admiral Komack at Starfleet Command while headed toward Altair, which is about 16.73 light years from Earth. Assuming that Kirk is less than 10 light years closer or farther than Altair, there is no noticeable time lag in a subspace radio round trip of 13.46 to 53.46 light years.
Since a light second is the distance that light travels in one second, there are 31,557,600 light seconds in a light year, or 10,519,200 times the distance that light travels in three seconds. For subspace radio waves to make a round trip of 13.46 to 33.46 light years in less than 3 seconds so there is no noticeable time lag, subspace radio waves have to travel at least 141,188,430 to 351,872,050 times as fast as light.
The neutral zone in "Balance of Terror" should be about 3,900 to 20,400 times as far from Earth as Kirk was in "Amok Time", or about 26,247 to 545,292 light years, and the neutral zone in "The Enterprise Incident" should be about 49.411 to 361.846 times as far as the neutral zone in "Balance of Terror", or about 1,289,016.4 to 17,004,712 light years.
There is no noticeable time lag when Kirk has a conversation with Commodore Barstow at Starfleet Command in "The Alternative Factor" near a distant planet. Barstow fears an invasion is imminent and when Kirk asks for backup starships:
BARSTOW [on viewscreen]: Negative. I'm evacuating all Starfleet units and personnel within a hundred parsecs of your position. It's going to be tough on you and the Enterprise, but that's the job you've drawn. You're on your own.
Since Barstow wouldn't withdraw Starfleet units from Earth, Vulcan, or other important planets, this planet should be more than a hundred parsecs, or 326.156 light years from Earth.
So the time lag is less than 3 seconds in a round trip of over 652.312 light years.
The neutral zone in "Balance of Terror" should be about 3,900 to 20,400 times as far from Earth as Kirk was in "The Alternative Factor", or about 1,272,008.4 to 6,653,582.4 light years, and the neutral zone in "The Enterprise Incident" should be about 49.411 to 361.846 times as far as the neutral zone in "Balance of Terror", or about 62,850,792 to 2,407,361,400 light years.
Those calculations provide some support for Spock's statement in "All Our Yesterdays" that:
SPOCK: That is true. I am not from the world you know at all. My home is a planet millions of light years away.
But in "By Any Other Name" Kirk says:
KIRK: What's the point of capturing my ship? Even at maximum warp, the Enterprise couldn't get to Andromeda galaxy for thousands of years.
So apparently there is some variable factor in the time it takes subspace radio messages to reach their destinations, and the Romulan Empire is probably much less far flung than those calculations indicate. But it could be spread out over less than a thousandth of the calculated distances and still be a vast empire ruling hundreds or thousands of intelligent species.
In "The Deadly Years" a part of the Romulan neutral zone is between Starbase 10 and Gamma Hydra, which is probably Gamma Hydrae, which is about 133.8 light years from Earth. In "Whom Gods Destroy":
SPOCK: Fascinating. What maneuver did we use to defeat the Romulan vessel near Tau Ceti?
KIRK 1: Very good, Spock. The Cochrane deceleration.
Tau Ceti is about 11.905 light years from Earth, so Gamma Hydrae is about 11.238 times as far as Tau Ceti.
So Romulan warships have operated over a distance of at least about 150 light years implying that the Romulan empire rules many different intelligent species considering how common they are in
Star Trek.
So there is no reason to believe that either the Klingon Empire or the Romulan Empire is a monospecies realm.