"How many times can you change?"

"507."

It might have saved time, but the Doctor regretted lying to Clyde.

507 was a nice number, but it was only arbitrary; in reality, the Doctor didn't know how often he could regenerate now.

After the destruction of Gallifrey... with all the energy his body had absorbed when every other member of his species was annihilated...

He had the physical potential to regenerate potentially as often as he wanted, but even with the TARDIS and the remnants of the Matrix available, he still only had a certain amount of mental space available in his mind to keep his old personalities after he'd regenerated...

And did he really want to live that long anyway? It sounded good in theory, but in practise... living so long after every one of his companions had been and gone...

Well, it wasn't like it was something he had to worry about any time soon; he still had quite a few old friends left for him to spend time with before that kind of problem became particularly relevant.

All he could do was wait and see what happened when the time came, and make some kind of decision then.