I don’t think there’s a set number that dictates whether it becomes the “ideal” conversation. No matter the topic, you need enough people to keep it fresh and interesting, and that could be two people or it could be twelve. If there are too few people, you run the risk of only one point of view being discussed, which is great for some things, not so great for others, and if there are too many, managing the conversation can feel like trying to herd sheep. So in that sense I think it also requires a topic that can be easily discussed between a few people or a lot of people. That way it doesn’t really matter how many people you have talking, it’ll still flow easily and feel comfortable.