The total number should be very easily available, but as far as I know, it's just not available to us.
You can click Find on the menu at top right of any WikiTree page other than in G2G, select Pending Merges on the dropdown list, then click the Unmerged Matches on the link near the top of the page. That will display the list of all current unmerged matches, however it lists 20 of them per page and does not show either the total number of unmerged matches or the number of pages that there are. It would be a nothing to display those numbers, though.