Robert:
Thanks for asking and for your interest in Wikitree.
There is no easy way to determine how many descendants are linked to a profile, but if you are willing to do a little work with your mouse, you can build such a report - five generations at a time.
Merely click on the "Descendants List" button near the top of the subject Profile which will report up to five generations of descendants for that individual. (You might like to print that page). Then on each of those descendants' Profiles you can repeat this process. It is somewhat cumbersome at the moment.
Thanks again, I hope this helps.
Keith