I've also been asked in interviews how many schools I applied to, and also how many I received interviews with. I knew it was a possibility but frankly, I didn't think it would happen to me.
But it happens and I probably didn't answer it as well as I could have, but I was honest with them and gave them an answer. To help myself feel like I didn't hurt my chances I still made sure to also reiterate my interest in the school and why I applied and felt I would be a good fit.
I do agree with Mr.Smile12's response and had I seen that before then I would have gone with something broader than exact numbers.
Oh, I know many interviewers who ask where else an applicant has interviews. Usually it is a way for the interviewer to figure out how competitive you are and if competing schools might be in play. However that also brings up the possibility of illegal collusion (why we don't get schools names of others who also extended offers). If they ask for numbers, it's a weird measure of validation of how competitive your application is.
The problem of course is that not everyone sends out II's at the same time, and thousands will receive II's later in the cycle, so it's not an accurate measure. Nevertheless, chances are if you are interviewing early, you probably have more than one school interested in you, so I would always tell interviewers to assume everyone they are talking to will have the opportunity to choose later on ("SO DON'T ASK THEM THAT!"

).
Again, how many sheets of toilet paper...?
