I'd rather interview and rank too many than be left to scramble. Of course, in specialties with more spots (FP, IM) you are more likely to match, where things like ortho or ENT would be a different story. I know several people who were decent candidates who had to go down to #7 or 8 on their rank lists, and I also know a few who ranked only 3 programs for things like general surgery who did not match.
Long story short, though you very well may get your #1, I'd be safe and interview at 10-15, rank 8-10 at least. And don't listen to PD's and chairs who say they will "rank you highly"- even if you are #10 on their rank list, if it is a program with 4 spots and they match people in their top 8, guess what? It's worth your time and money to overdo it and avoid the scramble.