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

Note that counting category views involves an extra UPDATE query on each category 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 categories it could become a significant slowdown which could motivate you to turn off the switch.