Generally one, outside of regeneration episodes. There have been specials - the Three Doctors, with the First, Second, and Third Doctors - the Five Doctors with the First through Fifth, although the Fourth was portrayed through a couple clips - the Two Doctors, with the Sixth and Second - and of course the 50th special, which you could argue had them all.There is no rule. It all depends on the imagination of the writer and (in the case of TV or audio) availability of the actors. "The Five Doctors" does what it says on the tin. The novel "The Eight Doctors" (novel) and the audio drama "The Light at the End" (audio story) feature the first eight Doctors. The anniversary audio book series Destiny of the Doctors features all 11 (not counting the War Doctor). The IDW Comics story "Dead Man's Hand" (comic story) features a cameo appearance by the first 11 Doctors and the War Doctor. "The Day of the Doctor" holds the record so far (i.e., before the start of Series 8); it featured 13, including both the War Doctor & the Twelfth Doctor, though most incarnations appeared only very briefly. There is no limit.