Charting the outcomes 2014 says the average contiguous ranks was 14.4. I don't know it that counts ranking 1 program twice for a categorical and advanced spot or not though. The curve shows almost 100% match rate if you rank 11 programs. Given that I would aim to rank at least 11 so I would expect that going on 12-15 interviews would be a good idea. That may be too many but I don't know.