Not a lot in actual numbers, IMHO, but one is too many.
I've never been a police officer but I worked as a security guard. It wasn't a career choice, just a job with hours I needed and they were hiring at the time and my kids were at home wanting to eat. I was not one of those cop wannabes that go into security work because they couldn't (thankfully) make it through the selection process.
Most LEO's would treat me like a brother officer and talk openly. Many quickly earned my respect. One treated me like I was no better than your average potential perp on the street (i.e. civilian), and one other I was convinced was actually looking forward to shooting someone. This last I am glad to report was Mcmanaging, last I heard.
Still some get through. The Field Training Officer program is supposed to weed out those, but then that doesn't catch all of them.
The "FTO" program is on the job training that is constantly supervised by trained officer-instructors and the trainee is evaluated on a daily basis within a structured program.
A good guy after a few years may be under so much stress and no one see's it. That guy could just go off, there's so much of the "taboo" in the world of macho that some aren't likely to admit a problem and go see the department shrink. Who knows what state of mind any of us are in at any given moment? Problem is, not one of us is perfect. I would hope though that the other cops on someone's shift would see what's changed in a guy and let the supervisor know about it.
Variables...variables. I think we call that life, it's a daily adventure and sometimes it just goes all wrong.