It depends on the school, honestly.
I've been to 3 interviews, this is my experience:
The number of questions depends on your interviewer. You might go off on a tangent. Most of the time in my interviews, the questions branched off of my answers. You can't really expect any set number of questions.
What does a typical interview look like? The interviewer asks you questions that have to do mostly with your character, personality, and relationship to healthcare (they already KNOW that you're academically capable if you've gotten to the interview stage). You answer them in a way that is honest, straightforward, and defendable.
The ones I've had so far have been more conversational than interrogatory (is that a word?), and have involved the interviewer asking me about why I want to go into medicine, what I've learned in my clinical experience, what my ideal practice would be, why I wouldn't rather do _____. Basically, know yourself. Know why you want to go into medicine, know what life experience has taught you that you do want to go into medicine, know why you don't want to go into research/law/education/extreme sports/etc. If you can't answer something, be honest about it. It's okay not to know everything, it's not okay to give false information or pull an answer out of your bum for the sake of not looking dumb (it really makes you look dumber).
They basically want to get a feel for you as a person, so as to better determine if you will: fit in with their school, make a good physician, represent their school well, be pleasant to be around.
I've gone in to each interview as a person, not as a candidate for admission, and it's worked well for me in minimizing my stress. If you concentrate more on knowing yourself and why you're going into this field, and less on what is expected of you, you will feel more comfortable and you will appear more genuine and "human" to your interviewer.
Do a search for "medical school interview questions" on google, you're bound to find something. Get a feel for what MIGHT be asked, so you'll have some things to think about and reflect on beforehand. Don't prepare answers to questions, but do prepare for interviewing. What I mean by this is: work on speaking clearly, eliminating words/phrases like "uh.." "um.." "and, like" "well... let's see" from your immediate vocabulary reserve, and standing up straight and looking confident (even if you don't feel confident). Find the happy medium between appearing wimpy and appearing cocky.