I would recommend that your exhibit, whatever the size, be of one consistent theme - that could be black and white photos, it could be one specific location, it could be a single topic, like cars or trees. In perfect circumstances, I would also make sure that they are all one medium, ie all black-and-white silver-gelatin prints. The more consistent your body of work being presented, the better your chances of someone liking it well enough to buy something. If you are all hodge-podge all over the spectrum, people are less likely to notice your better images, and less likely to take your work seriously. This becomes even more true when you have fewer images to show. If you are only going to exhibit five or six at a time, you should very carefully edit the images to display so that they are similar in size, medium, and content.