I think the most I had on one forum at a time was around 5000 or so visitors in a single hour. I believe it was from a giveaway we hosted years ago, where we got a ton of signups, entries and so on. Sadly most of them weren't our users and just signed up for said giveaway, in turn resulting in not many people sticking around. Which is another thing, giveaways aren't good for gaining traffic, or keeping them around.