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?
Wonderlost 
It would give way to a number of comedic situations! 😉 But I hope they don’t stop at No. 13. In the Sarah Jane Adventures, Matt Smith said Time Lords had a much larger number of regenerations. There is an article here: http://www.theguardian.com/tv-and-radio/2010/oct/12/doctor-who-immortal-reveals-bbc
Everyone sure hopes that the Doctor does not end at thirteen. I guess that is the main reason why they ditched the previous notion that Time Lords can only regenerate twelve times. 🙂