He chuckles, but there's no amusement in it. It's a wry and dry sound. Because that reminds him of what it was like.

"If we're too injured to regenerate and we still have regeneration energy inside of us, every single cell in our body will keep trying. Dying properly takes days. Sometime weeks. Unless we force ourselves to give in. I know of only a handful of Time Lords who accomplished that. But regeneration energy has been limited. So, there's enough for twelve regenerations. Thirteen bodies and then... no more. You simply don't have any more energy to use."

Unless a thing happens and you're granted more. But he'll not get into that right now.