...interviewed at 4, and was accepted at 4 (one of which was Iowa!) In retrospect, this was probably a bit on the risky side. This was also a few years ago, it may be a bit easier or harder now.
I think 15 is plenty. No offense, but if you don't get in to 1/15, I really doubt you'd get in to 1/45 or whatever other number you'd apply to. I think MSTP admissions tend to be a little more predictable than straight MDs, as has been said before on this forum. If you have high GPA and MCAT and a great LOR from a research mentor, and lots of research experience, you will get interviews most places; then it's up to you to not be a tool 😉 This is different from straight MD--I think they are more likely to be looking for "that special someone" who has decent numbers but also that intangible special something which is difficult to predict (20,000 volunteer hours, grew up in poverty in Central America, Olympic Athlete, etc.)
Good luck!