In universe, Starfleet was both refitting older ships and launching new ships, simultaneously and continuously over the last several years. Hence, the disorder in registry numbers. Stone's list showed the status 10 Star Ships in service at that time (Stardate 2947.3), while the "list" was increased to 12 in TIY (Stardate 3113.2). The Starship fleet increased by two ships in the ~166 stardates or about 2 months. Maybe Kirk was including ships in drydock nearing (re)launch, too. If Starfleet was in a rebuilding program, then I can see Starfleet bringing on-line two or more Starships per year. Was the fleet of starships nigh destroyed during some Klingon war a few years before the TOS, and we are seeing the regrowth of the starship fleet during TOS? (Kirk got promoted and medals for something big.) Is Starfleet dragging old ships (Constellation, Republic, other 1600 number ships) out of mothballs and refitting them to boaster the fleet? Reagan dragged four battleships out of mothballs and refitted them to boaster the US Navy to meet their 600 ship goal. Using my "strike-through" technique: Taking old Starbattleships out of mothballs and pressing them back into service is "the quickest and most cost-effective way to get more . . . naval starfleet muscle to sea space in the mid-19802260s," according to Navy Starfleet Secretary John F. Lehman Jr., who is bent on creating a 600-ship fleet to challenge the KlingonSoviet Union's burgeoning naval space fleet might.YMMV.