There's also the issue of need
to contend with as well. Given the size of the Federation, one might argue that Starfleet should have somewhere close to a million or so ships. But if Starfleet can cover it's responsibilities with much fewer vessels, then it's likely to do that as well, IMO.
I don't think there's any way to adequately defend any area spanning thousands of light-years unless you simply fill the skies thick with vessels. But I get the impression that a single starship covers quite a lot of territory and that large numbers of vessels are only needed during times of war.
In such occasions, we may see Starfleet rely on everything from previously mothballed older vessels to even "kitbash" ships culled from multiple designs to increase its numbers dramatically in a hurry...