I would say over 100, I know lisa has tryed a forum several times and lacked real traffic on it, but I think its definitely succeeded now. Sign up for Google analytics's if you haven't already, and look at your page views, and bounce rate.

http://www.google.com/analytics/

If your bounce rate is reasonably low and a lot of people view your sites content and don't just leave at your homepage, and have a decent amount of visitors I think they are all the factors you need for success using a forum.

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