We can have as little as 6 people, or we've also had enough people to go onto 2 large tables at the cafe. I never know until the day. There are people who don't turn up, things that happen last minute and also people can come unannounced.
No matter the number of people, I've found that we always have an enjoyable and productive morning. The bonus of a smaller group is that I'll even get a chance to join in in the conversation. In the case of a larger group, I usually go around and try and make sure that everyone is getting a chance to meet different people.
After your RSVP you will get an email the night before giving you a guide to how many people I'm expecting.