5
Just click on your forum, and at the bottom of your 20 or so topics are three drop down menu's. The one on the far right has options to show a selection of topics, for example, the last 20 posts, or the last 50 posts, or the last 100 posts etc etc.
Just choose which one you want, and they will all be shown. I think there's also a setting in the module preferences for how many topics to show by default, although I could be wrong.
Hope that helps.