Count up the ship classes we see by the end of Voyager. At least this many types are in service (canon only)
In addition we see a few classes during the DS9/TNG run that could logically still be in service:
You could also reasonably add the Luna-class (Titan).
That's 20 different kinds of ships! And that's assuming the Shelley, Centaur, Yeager, etc. we see were simply forced into service and decommissioned after the war and don't really count. Or that the apocryphal Constitution, Constellation sightings in the DS9 run weren't actually there.
And what's more, a lot of them appear to have overlapping purposes. The Sovreign and Akira are big fighting ships, the Steamrunner, Intrepid, Norway, New Orleans, Prometheus are all smallish tactical ships and all roughly the same size. Nova and Oberth are both science/scout ships, etc
For that matter, the Nebula's very existence bugs me. It literally has all the exact same parts as the Galaxy, just re-arranged. Why would they even do that? It can't be cost effective and it doesn't hold any aesthetic value. You could literally build a Galaxy class ship for the exact same cost as a Nebula (probably a little less) and not have to devote any extra resources to R&D to reconfigure everything.
Obviously some of this has to do with older ships staying in service for a long time while newer ones are developed, but that doesn't explain away everything. Why would you build Steamrunner/Sabre/Norway classes all at the same time when they are essentially the same kind of ship. Just pick one and build three times as many instead of squandering resources to develop and engineer more and more classes.