I'm an MS4, so take my opinion with a grain of salt. That said, I would assume that the "magic number" is completely dependent on how competitive you are as a candidate.
I'm honestly confused by the whole "magic number" idea to begin with. On one hand, I've repeatedly heard that 10+ is what you should be shooting for. On the other hand, every single EM applicant from my school has matched into one of their top 3 choices for the past 2 years running (as a point of note, I go to a middle tier med school).
Obviously those people had more than 3 spots ranked and had the benefit of finding those top 3 among a larger interview pool, but I seriously doubt their results would have been measurably worse if they interviewed at far fewer places.
TL;DR: I have no firsthand experience to back this up, but I'm guessing that the whole "magic number" idea is really only applicable if you're worried about the strength of your candidacy.