1: Kirk said "there are only twelve like it in the fleet" obviously implying starships like the Enterprise.
Well, let's explore that a little.
Kirk's statement is just that, "there are only twelve like it
in the fleet". That doesn't mean only twelve starships in the fleet, just twelve like it.
Now what about the other ships?
We know from the diagrams visible in "The Enterprise Incident" (and perhaps one other episode) of a version where the saucer lower edge is beveled, secondary hull is rounder and the nacelles offset differently and thus not like the
Enterprise. I like to call this the pre-Captain Pike ship, or what the Enterprise looked like when she first rolled off the assembly line.
So now you have 12 ships, like the Enterprise and a number of ships not like the Enterprise, yet still sporting a similar configuration.
And if we got nit-picky, a third version exists, the USS Constellation from "The Doomsday Machine" since the model kit from AMT is not built or shaped exactly like the Enterprise or the pre-Captain Pike ships.
That's only just using exclusively TOS info