Well Rick S. Gave the real length which is 478.55m so I played with the hight and I got 126m Then Rick S gave the hight he use for making the cutaways, which is 13 feet. 126m to feet is 413 feet. I get 31.7 decks. Now you could round down to 31 or even 30 decks.
The deck count could jump up and down depending what the ship needs. Now you can have tanks at the bottom of the ship, that takes up the space of 1 1/2 decks or maybe two decks, which would look like one large deck. Also the jefferies tubes, maybe the ones on the Enterprise-C are larger then the ones on the Enterprise-D or B. That too could give you a low deck count.
I love even numbers, so I would go 30, but I have no problem with 31 or even 32.