My view is two 20A (standard sized circuits should be fine). What you need to make sure about is cooling the room. Most of the electricity ends up as heat. I've been to a fair number of basements that have five or more machines, and by the end of the night you could pop popcorn by simply sitting it on the table. Cooling/venting is almost more important than the electrical. It is pretty typical to have two circuits for a large room, but much less typical to have enough cooling to run 10 hair dryers and not start pitting, badly!!