Actually the 13 regenerations is not canon, the Doctor has unlimited regenerations. The 12 was a throw away line from the Doctor Who film and for some reason it has really found legs. I wouldn't worry about the franchise sticking with a one off comment. While the programme is good and has high ratings they will continue to regenerate, part of the beauty of the show....

As for Matt Smith leaving, sadly I won't be disappointed, he has been one of my least favourite Doctor's, just below Colin Baker. Saying that I think he's much much better with Jenna Louise Coleman. I can't see Stephen Fry happening, he's a little too high profile for the role, same for Hugh Laurie. Look at British household name actors, not necessarily well known in the North American markets. Someone like James Nesbitt or of that ilk anyway.

I know that Fry is just a pipe dream, I just think he would be a really good Doctor.
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I know, but he would be great, at least he would if he could run! I do think Hugh Laurie would be even better as he can really do all aspects of acting. I remember seeing him in early episodes of Spooks and gawd he was a villain... and he was on our side!

The number or regenerations came up in an episode when Peter Davison was the Doctor.

The episode was Mawdryn Undead. He was asked to help a group of eight aliens stop their perpetual regenerations by giving up his remaining regenerations to kill Mawdryn and his fellow sufferers. The Doctor said something about eight regenerations and eight of them.

So since he was number 5 (resulting from the 4th regeneration)... 8 more regenerations would make 12 total regenerations and 13 Doctors total.

I don't think the 507 was meant as a a serious number. I could be wrong... but over the past 50 years (or at least since the late 1970s) 13 has been deeply ingrained into the Who Universe...

Exactly... even when you look at individual episodes, most of them have huge logic holes that make no sense LOL You only have to look at The Big Bang and try to apply logic to the Doctor escaping the Pandorica to realise it's a fruitless task!!

Actually when addressed about the regeneration limit, Stephen Moffat said, and I quote, "It's Doctor Who...we'll figure something out." Seriously, they contradict themselves all the time, and that's the wonderful part about writing for DW is that you can come up with reasons why you contradicted yourself and no one would question it!