You basically can't give people numbers or statistics because invariably people will try to make that number as big as possible.

I don't think I've used any forums that at least publicly surface structured "reputation" although of course informal reputation exists everywhere, including forums.

Post count yes, that's pretty common. But if the forum you use is any good, they'd actively combat posts/threads that aren't actually contributing to the conversation.

One of the biggest and most active forum in Sweden is actually pretty good at this, probably mostly thanks to its ~100 iron fist moderators who do such a great job with cleaning up posts that aren't contributing to the discussions at hand. That it also have a really extensive set of rules also helps.