We have been running occasional house concerts for a few years. We can probably comfortably fit 30 people and squeeze a few more. So far the biggest audience we had was 18. I just warn perspective performers that they will not get rich that night. They get the entire gate and sell CDs. Since we are between New York and Phily, I suggest it as a fill in between bigger concerts on a tour.   Run a couple of concerts and see how you like it, as suggested above. When you start to get close to capacity, which probably will not happen for a while, suggest that people make reservations.