Starbase 12 was located on a planet in the Gamma 400 planetary system. The base was located close to the planets Achrady VII, Ceti Alpha V, Gemaris V, Mab-Bu VI, Pollux IV, and Risa. In 2167, Starbase 12 was under the command of Admiral Uttan Narsu. (TOS: "Space Seed", "Who Mourns for Adonais?"; TNG: "Captain's Holiday", "Power Play")
Later that year, after the USS Enterprise had been stopped by an unknown force, Captain Kirk ordered Lieutenant Nyota Uhura to relay the position and circumstances of the starship to Starbase 12. (TOS: "Who Mourns for Adonais?")
In 2364, the starbase was inexplicably evacuated for two days. Starfleet Command gave no explanation for this action. Captains Walker Keel, Rixx and Tryla Scott saw this as further evidence of a conspiracy within Starfleet. (TNG: "Conspiracy")
In 2369, when Lieutenant Aquiel Uhnari and Klingon commander Morag were believed to be coalescent organisms, these two were supposed to be brought to a secure medical facility on Starbase 12. (TNG: "Aquiel")
Presumably, Starbase 12 was located in the Alpha Quadrant.
Starbase 12 was also featured in the video game Star Trek: Bridge Commander and the novel Ship of the Line, in which the USS Bozeman protected Starbase 12, which was the building site of the USS Enterprise-E.