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?
Thats the thing. We think say 20,000 is high but in a 8000 light year area plus more outside exploring that really is not much!
When dealing with such vast amounts of space, high takes on a new meaning.
Not when if you think about it the majority would be auxiliary craft that would not be capable or have limited combat capability so would not be first responders anyway.