The Federation is a huge conglomeration, and need ships.
Now, lets say they made 1000 miranda's from 2275 to 2295, like most ships and airplanes there would be different "Blocks" like a F-16 Block 60 is the current build even though the F-16 as a platform has been in production for over 40 years.
So as the ships, not just the Miranda, age, the get the occasional refit, new power systems etc. but some of the older block 10's and 20's you can only refit so much, so as they age out, they are given to individual planets space navy's, something like the National Guard or Reserves. There still servicable as ships against pirates, border ships etc. An Old ship is better than NO ship.
Now the latter blocks of Mirandas are still serving in the main fleet, doing, whatever there doing, but when the WAR came along, all the individual planets navy's donated ships to bulk up the fleet, of course keeping some for there own protection.
As for Crew, there was alot of automation, so you may have a crew of at most 50? Enough to do some crew rotations, but if something really broke on it, its out.
The ones comeing from the reserves were probably "Refreshed" at a space dock, given as many new systems and automation as possible, upgraded weapons, like being able to fire quantum torpedoes, etc.
So to me, thats why you see a lot of Miranda's and excelsiors because there older "Mothballed" or Reserve Fleet ships that were reactivated when the war started. Some of the older ships that might have more damage or old age, but still had servicable parts, were converted over to the frankienstien fleet.