I believe that is how many students they have graduated from their class since becoming a mentor! Pretty cool, eh?
I don't think it's the number that you have graduated because I have two students right now (my first two students) both are still in class with me so they have not graduated yet, though the number 2 is still beside my name in my profile. Maybe it is how many students you have had overall? Or how many students you have in class right now? I really don't know lol..
Near as I've been able to tell it's the total number of students you've ever had that have gone through the class. Canceling a class removes a number, taking on a student raises the number, and graduating and failing should leave that number the same.