At Admin Panel -> Settings -> Switches, there's a "forum views" switch. When activated, this simply counts the number of unique visitors who have browsed each forum. The total is displayed by the forum in the list. To display the number of views for a forum anywhere within the scope of a forum or in a forum toplist, use {CATVIEWS} -- or {CATVIEWSFORMAT} if you want the number formatted with a thousands separator.

Note that counting forum views involves an extra UPDATE query on each forum page which counts as a view. In most cases this won't slow anything noticably, but on a site with tens or hundreds of thousands of forums it could become a significant slowdown which could motivate you to turn off the switch.