I haven't forgotten that. But look at the list of starships in TOS: the USS Constellation, the USS Defiant, the USS Enterprise, the USS Excalibur, the USS Exeter, the USS Hood, the USS Intrepid, USS Lexington, and the USS Potemkin. (We assume that the Intrepid was a Constitution-class vessel, without even actually seeing it in the "original" (pre-remastered) TOS. Does anyone have a list of carriers to match up with these names, and others to fill out the balance of Kirk's 13?
One problem I see with the only-13 Constitution-class theory is that of the vessels listed above, the USS Constellation, the USS Defiant, the USS Enterprise, the USS Excalibur, the USS Exeter, and the USS Lexington all appeared to have human captains during their appearances in TOS, and the Defiant, Enterprise, Exeter and Lexington appeared to have either mostly- or all-human crews as well. We did not see any non-human captains of starships during TOS, although we're told that "Intrepid is manned by Vulcans". So if there were only 13 Constitution-class vessels in Starfleet during TOS, and Travacious' list above of losses of ships and crews from this class are that serious, the peacetime Federation is suffering wartime losses. None of the Enterprise's officers, or any Starfleet guest characters seemed to have a commensurate sense of urgency about them.
The only thing I would surmise (based on no clear evidence) is that if Starfleet had only 13 Connies during TOS, then Starfleet must have a much larger pool of "Connie Lite" light-cruiser-style ships that would be almost as capable as the Enterprise, but somewhat less massive and with smaller crews, like Starscape's Spitfire. (additional views: 2, 3)
Otherwise, Kirk's 13 would refer to the historical group of the original Constitution-class, Constitution-subclass prototypes/pathfinders/first-production-run, allowing for subsequent subclasses of Connies like those listed in FJ's Tech Manual.
Twelve. "There are only 12 like it in the fleet." Not "there are 12 MORE like it in the fleet." That's 12 including the Enterprise.