Does anybody have any idea how many ships Starfleet had in commission during the Enterprise, TOS & TNG eras?
I got the impression that Starfleet during the Ent era only had a few dozen ships or so at best. We saw several smaller Starfleet vessels (like the Saratoga) in addition to the NX-class.
TOS? It's anyone's guess. If there really were 12 Constitutions (which I really can't believe given the events of The Ultimate Computer), then there still would undoubtedly be numerous smaller vessels. You can't defend the Federation with 12 ships. If I had to guess, I would say the TOS-era Starfleet probably had a couple of hundred ships at least.
TNG-era would have had several thousand.