Which is less than helpful in establishing Starfleet numbers, but at least it rules out the very high ones.
Not necessarily. Space is big. A single sector is 8000 cubic light years! The Federation is made up of a lot of sectors, and Starfleet operates even outside of that. A hundred thousand ships might not be near enough to do everything Starfleet is supposed to do.
Of course, there might be 50k Oberth-class ships making up the bulk of Starfleet, who knows?