the Master had reached the end of his life in the classic series (meaning twelve regenerations). He stole the body of Tremas, was exterminated, stole another body and died again. He was ressurected by the Time Lords in the Time War, but turned human and fled. Presumably he was given a new regeneration cycle when he regenerated. It is unknown whether the Yana incarnation was the first in this new cycle. "Last of the Time Lords" suggests he could regenerate again, so he has regenerated between 13 and 23 times. He has also posessed two bodies, died and been ressurected twice, and turned into a snake thing after extermination. Only one regeneration was on screen.