Uhmmm, not very many. Most have a Mac for doing audio or video, or people who have just converted to an IPhone and are on the apple bandwagon. They just want 100% Mac because they think they are better. The last group will install the odd game that was ported to OSX.
I have some friends that now own IPhone, a MAC computer, and Apple TV. They are 100% MAC and it irritates me.