I think most people will agree that there is no benefit to ranking more than 11 programs. There is an oft cited graph that was in a paper published about the Ophthalmology match a few years ago. I've been told that 11-13 interviews would be a safe number to go.
I'm impressed by people who manage to schedule 21 interviews! That's a lot of $$$ and time involved. In any case, so many of my interviews overlap and I'll end up having to cancel a couple because of scheduling conflicts.
I would talk to your advisor. Better to concentrate on a reasonable number than to go to so many and be incredibly burnt out at the end. My 2 cents.