The Trek universe is hard to keep straight. In TNG, Shelby states it will probably take a year to get Starfleet back up to scratch after the losses at Wolf 359. By the Dominion War, it appears the Federation are losing about that number of ships in an average week (at the Second Battle of Chin'toka the alliance lost 311 ships, including Klingon and Romulan forces, but that does mean in probability that's AT LEAST 100 Federation ships in this single engagement).
In the Wolf 359 continuity, then yeah a possible total build over the decades of 20-30 Connies, with perhaps a maximum of 15 being in service at once is quite probable. In the Dominion War continuity, then that number does seem rather small.
We also have the problem that in TOS the Starfleet is almost certainly smaller than in the later shows, and by TUC and the Khitomer Accords they are actually planning to reduce the size of Starfleet. But when we get to TNG it's larger again so there's a lot of chopping and changing as to how big the Federation and Starfleet is supposed to be.
In the Wolf 359 continuity, then yeah a possible total build over the decades of 20-30 Connies, with perhaps a maximum of 15 being in service at once is quite probable. In the Dominion War continuity, then that number does seem rather small.
We also have the problem that in TOS the Starfleet is almost certainly smaller than in the later shows, and by TUC and the Khitomer Accords they are actually planning to reduce the size of Starfleet. But when we get to TNG it's larger again so there's a lot of chopping and changing as to how big the Federation and Starfleet is supposed to be.