Mentors can have two roles. By default, they are randomly assigned to newcomers. There is an option for users who only want to be mentors to people from events or classes: they don't get any newcomers auto-assigned to them, but they can claim them manually.
The ideal number of mentors is: one mentor for each group of 500 new accounts, with a minimum of 3 mentors.
Know more about how mentorship works
