Do you mean how many more interviews to expect to come in or how many programs you should interview at to match?
For the first question, I applied to about the same number of programs maybe 33 or so. By this time last year, it looks like I had 19 interview offers based on the sticky. It looks like I got another 5 or 6 after this time (10/9). I didn't post super late offers. I think I ended up with like 27 interview offers out of like 33 applications. Assuming the timeline is similar this year, I would expect that the trajectory would be similar and the best way to predict your number is based on how things are going for you at this point. It sounds like we had fairly dissimilar applications so I'm not sure how much help my experience is. I had similar scores, but went to a generic state school, had graduate degrees and so on.
I also applied to the IM-PSTP programs. So about this time last year, I was accepting those. As more Med/Peds offers came in later from strong programs, I started dropping earlier scheduled interviews. I would imagine you won't need IM as a backup unless residency rankings is important to you. I know applicants who were much weaker that got into good programs last year.
For the second question, I think most people that I know matched between numbers 2 and 6 on their rank list. That is probably highly biased by the people I now talk to the most. Just guessing, since the Med/Peds programs at the world famous institutions (MGH, BWH, JH, Penn, UChicago, Duke, etc.) tend to be very small and the programs at very good institutions (BCM, Mich, OSU, IU, Minnesota) tend to be significantly bigger, I would imagine that not getting one's first choice is extremely common. By contrast, it is sort of the opposite in IM because the big name programs also tend to be the biggest programs.
I hope it goes well for you. Interviews are a lot of fun in retrospect! Though tough at the time.