i "audienced" at galleries in boston and new york several years ago. the advice i was given by friends who were involved in "the scene" said 15-20 images. if there are themes - group them together, and start & end with your strongest pieces. "leave them so they are wanting to see more."
Thanks John. You've got bigger cajones than me. This old country boy would be totally intimidated by just the thought of trying to sell his wares in Boston or New York.