Teacher Discussion Forums

I'm thinking of starting a drop-in lunch time program for conversational English. It would be in a restaurant in the center of town that people working nearby could come and join anytime between 11:30 and 13:30. How many people would you think would work as a maximum? I think advertising that it would be a smallish number would be attractive to people who don't want to go to a class with 20 others in it, but of course I want to have enough people in it to make it worth my time. What number would you recommend?

For me, the headache of making a maximum would probably be worse than the fact that some people would be scared off by a large group. Let all of the students come unto thee.