Which is why there has been rumours around Dr Who actually having a "lost" son so that the BBC can carry on producing for another 26 (or so) regenerations from the 2.
The Master was granted a new body.
And urm.... poeple have been saying his son is taking over 'Harkness' << I doubt it. (AAND don't forget no matter how young John Barrowman looks, he will still get older.)
And... other people have been saying that Auntie Been and Uncle Russell could totally ignore the rules and give him as many as he likes.
He could be different because he is "Half human"
OOOOOR.... they could just end the show at 13.