What An Actor Needs In A Headshot

I'm about to get my first set of headshots done, but there are some specifics that I'm not sure of. Firstly, what is a contact sheet? Secondly, these 8" x 10" prints that every actor should have, are we supposed to have a whole stack to hand out, or are we supposed to have one to take to auditions but still keep for ourselves? It sound very expensive to keep handing out these 8" x 10" prints. Any other information on headshots would be helpful. Thanks!