About regenerating -

The original series was only supposed to last a couple of months, but was extended due to massive viewing figures and public interest. William Hartnell ended up playing the role for three years, and finally had to retire due to poor health. The regeneration idea was probably thought up as a way of carrying on the show with a completely different actor. And you can't deny it worked, as the show went from '63 to '89 with seven different actors in the role.

As for how many regenerations the Doctor gets, it'll probably turn out to be extendable in some unexpected but highly-convenient form.  ;)