Is there a minimum number of pieces to show or is just one enough?
One is usually enough, depending on the mood of the zombie processing your request, but the more the merrier! As for your other question, it's impossible to say. I'd just apply, the worst thing that can happen is that you'll have to apply again later with something newer.
@Franklin: Cool! Start posting your stuff and the chicks will come. Oh yes, they will come.