Yes, I have seen documentaries and read quite a bit about it.
Yep, people say it, and the favored theory is they rarely stop.
Nonetheless, whatever documentaries say, or what's written in books, or whatever LE or criminal psychologists say, it's logically impossible to quantity how many serial killers decided to just stop on their own without getting caught. We can estimate, but that's just a guess.
If they stop and are are never caught, and we didn't learn about them posthumously, then nobody will ever know about them. Just no way to know, deterministically.
I'll get that, insofar as the cases we are aware of where they have eventually been caught or discovered posthumously, we can see that most never stopped. But again, that's not proof that there haven't been numerous serial killers who eventually just stopped on their own.