To show total comments rather than total posts, edit the Member List info and in Member List Rows change {total_combined_posts} to {total_comments}.
You can also modify the memberlist to show different total amounts of users, by modifying the URL segments.
http://www.yoursite.com/index.php/member/memberlist/0-total_posts-desc-50-0/
You see that I have added another segment on the end there. That modifies the default values. In order this is what they do:
* group id (0 is all)
* order by
* sort order (desc/asc)
* number to show at a time
* offset
Category:Members Category:Templates