A huge range. I have some friends that did as few as 2 or 3 if they had to stay in a certain city with a spouse in a less competitive specialty like FP. (Like folks that wanted to say in IC would just interview at U Iowa and Cedar Rapids or something). Folks going into really competitive specialties would do up to 30 interviews. 😱
I did 8. I initially applied to 25 places, was fortunate to get interviews everywhere, and then pared it down. I wish I had gone on a few more. I felt real confident on the interview trail but programs lie to you. And there were programs out of the 8 I didn't like, even though I thought I would. When it all shook out I REALLY REALLY wanted to end up at UVA or U Mich, which fortunately I did. (Those were my top 2). If I had dropped to #3 on my list, things would have been fine, but I would have been disappointed. It would have been nice to have a few more programs that I felt really good about on there.