Twelfth Master's regeneration

Following his twelfth regeneration, the Decayed Master sought to survive beyond the end of his initial regeneration cycle. (TV: The Deadly Assassin) Shortly before he successfully possessed the body of Tremas, the Master claimed to be nearing "the end of [his] twelfth regeneration", (TV: The Keeper of Traken) which was later reiterated by the Fourth Doctor. (TV: Logopolis)