I agree that backup schools are entirely in the context of the individual applicant, but I definately disagree that lower ranked, lower average accepted stat schools are harded to get into because of number of applications.
My advice is take an honest look at your application with the help of other people, like a pre-med advisor, for example, and then apply to schools in a range around your level, including a few you feel may be out of your range, and definately a few or more below where your stats are. I found this played out amazingly by the book for me. I applied to 12 schools. Four that I didn't think I had a chance at (no interviews from all 4), 4 that seemed to be about my range (3 interviews, 1 acceptance), and 4 that I used as my "back-ups" (4 interviews, 2 acceptances). I was also on 3 waitlists when I decided on my school. So yeah, there are back-ups, for me they were SLU, Creighton, Finch, and my state school.