The rule of only 13 lives for The Doctor was first said in 1976 in The Deadly Assassin, featuring Tom Baker as the Doctor in his fourth incarnation, and revealing for the first time the regeneration limit.
According to The Guardian, this will be revealed in Sarah Jane Adventures episode ‘Death Of The Doctor’, which is set to broadcast on October 25th and 26th.
The Guardian said:
While the Doctor and Clyde Langer, played by Daniel Anthony, are in the process of outwitting spooky vulture undertakers the Shansheeth, Clyde asks how many times he can regenerate. The Doctor indicates that there is no limit. The action continues.
The BBC have refused to comment and said, “We never comment on future storylines for Doctor Who”