Number of visits allows you to be more specific on when, what, how, and with who the visit was with than milestone visits!
You can specify the status of the visit; late cancel, no show, check-in, absent, etc.!
These behaviors can be tracked over a time frame as well, and you can filter on the instructor, credit used, membership used, and class time.
Use case:
Creating an auto campaign to upsell someone once they sign in three times using a class pass within the last month.
Creating an autocampaign based on clients who no show their first ever visit.