allsprouts answered:
The first one, you only get twelve generations. The master has been defying the laws of timelords and has had a few too many :P
merps answered:
Thirteen times sounds better to me, simple and not a mouthful.
lauranicuspond answered:
thirteen lives. (Unless the Timelords give you a whole new life cycle, or you somehow manage to circumnavigate that like the Master did…)