When registering as a mentor in most programs, you can set the maximum number of mentees you're willing to work with.
If you've already registered, you can edit this number in the Profile page for each program you're enrolled in. Find the program enrollment, then click edit:
This number is not a guarantee on the number of mentees you'll be assigned. Results will vary depending on the type of pairing process your administrator runs, and how your profile aligns with the mentee pool.