Is there any way to reset that count?
in phpmyadmin...run this query
update mybb_cache set cache = "" where title = "mostonline";
That should work...
On phpmyadmin there should be a sql or a query tab at the top of the page click that and type in what labrocca said
update mybb_cache set cache = "" where title = "mostonline";
I also believe that there is a section in the MyBB ACP that can do that. I think it is ACP> Stats > Recount
There should be one that says reset most users online.
I don't think you can clear the most online using that feature.