True, there are more than 4 Nebula class ships that show up on screen through TNG, and those ships while not scale 6, are definitely close. Both the Ambassador class and Nebula class are scale 5, and there is not a lot of info on how long the Ambassador class was active, or how many of them were produced.
There are only 2 scale 6 Federation craft, the Galaxy and Sovereign class. We know that there were few Galaxy class ships made, and it is assumed that the same can be said about the Sovereign. The Galaxy was made more with exploration and peace-keeping in mind, while the Sovereign was made more with combating the Borg and other threats.
If your campaign is set in the year 2371 or on, then there is a possibility there will be some Sovereign class ships out there, as well as Luna class explorers which are also scale 5. (2372 projected)
This may help you out:
Scale 3 Starfleet Vessels: Daedalus, NX Class.
Scale 5 Federation Vessels: Excelsior.
Scale 4 Federation Vessels: Centaur, Constellation, Constitution, Hermes, Miranda, Sydney.
Scale 3 Federation Vessels: Oberth.
Scale 6 Federation Vessels: Galaxy, Sovereign.
Scale 5 Federation Vessels: Akira, Ambassador, Excelsior, Luna, Nebula.
Scale 4 Federation Vessels: Centaur, Constellation, Constitution, Hermes, Intrepid, Miranda, New Orleans, Norway, Olympic, Steamrunner, Sydney.
Scale 3 Federation Vessels: Defiant, Nova, Oberth, Saber.
These lists were made using the core book and Command Division Supplement. I sorted them into general years. (Ent: 2140-2175, TOS: 2220s-2300, TNG: Any vessel made after 2230. Some in the TNG list may have been decommissioned or otherwise no longer used, however most have been seen at one point or another in the series/movies.)