This whole thing is RIDICULOUS! ... Time Lords can ONLY regenerate 12 times
—
Science progresses and things change, so its easy to come up with an explanation, and I seen to remember there was something about that in one of the shows - I don’t remember which.
The Master always come back - as he always has backup plans in case the current one fails. In one episode his consciousness was inside a ring, and he managed to takeover the body of the wearer.
The ridiculousness is part of the fun.
The new series had/has NONE of that and sol, was NEVER as good and just kept on getting worse and worse and then GOD AWFUL! I gave up on it with the last male Doctor and haven't watched it since.
Nope...do NOT play games with the sex of the Doctor, nor the prescribed number of regenerations!