this question is illegal, and it really upsets me. i usually answer quite vaguely, but some places have made me specify schools. at ucla, my interviewer even made me list every single school i applied, where all i've interviewed, and where all i've been accepted. i was so upset. that is none of their business. it is completely irrelevant and really annoys me.
i guess just say whatever you are comfortable with. if you don't mind sharing, just list as many as you want until it looks like they don't want to hear anymore. otherwise, be vague and see if they'll let you off the hook with that.
personally, after the interviewing season is all over, i'm going to email all the admissions offices and tell them if i had an interviewer ask that. it's illegal, and i want to make sure the adcoms know they have people asking this sort of crap.