I think the rule of thumb is about 10. If u look in Frieda, u can see how many people were interviewed last year for that program. From my searches, I saw that for every resident, they interviewed 10-15 people.
I also asked a few residents, and they said to be secure, around 10 something is good. This obviously depends on where u apply and how competitive u are and if u made good impressions, etc. The point is, assuming u're in the same applicant pool as u're peers for those programs that u are interviewing, interview/ranking 10+ is pretty safe, though obviously not a guarantee.