system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] How do Time Lords genetically limit regenerations?
gpt
We get conflicting information but it seems that the regeneration isn't an inborn ability, but something they're bestowed with- possibly part of the process that makes them for-realzie Time Lords instead of, well, ordinary Gallifreyans.

So they control the number of regenerations by only giving you that many in the first place, and can give more if they choose to (and everyone in charge of deciding to, decides to). As for why they'd bother putting a limit on immortality- it might be a failsafe to prevent society from growing too stagnant from having the same people in charge for eternity.