It was established in The Deadly Assassin (1976) that a Time Lord can regenerate twelve times before permanently dying–a total of thirteen incarnations. In the 1996 television movie the Eighth Doctor explicitly said that a Time Lord has “thirteen lives.”
Wouldn’t it be fascinating that the Thirteenth Doctor (presumably the last incarnation) be a blonde female, just to break the stereotype toward blonde females being stupid and all? Just a thought I had today. What do you think, guys?