Probably a lot. Such rarely hit the news, though, do they.
We had an interesting case here where a security officer of a private security firm was videotaped kicking a handcuffed bloke while the other security officer was pinning him to the ground. Caused a little bit of a hassle, too. There are claims that it's not very irregular, but it's not very often that it gets videotaped by someone (obviously, the security cameras are the same firm's as the security monkeys).
I can't remember ever hearing of a police being accused of such, though. But those are probably effectively swept under the carpet.
Oh, it's still on youtube.
Here.