Slashdot did this (maybe still does). I don’t know the exact mechanism for determining how many. They also had meta-moderation where they’d give you a list of posts and votes and ask if you thought the voting was fair. Doing that was supposed to make it more likely you got votes. I got them regularly and then one day never got them again. I suspect it was because I “failed” in the meta-moderation voting, but I had also been critical of the site and there were rumors of a blacklist.