This is an easy number to find in Reunion. On the file menu select "List" and click on "Last Names," choosing "All People." This will pop up a new window that details your surnames.
I have 555 unique surnames in my family tree of 3148 people (an average of 5.6 people per surname). Of course, it's counting surname variations as different names (ex. Utz / Ouzts). My top surname is actually "Unknown," with 194 instances. Everyone in my tree has a surname, so if I don't know it, they are "Unknown." Ignoring that, here are my surname stats: