How many people required for a good conversation isn’t really based on the number of people talking, it is about the number of people listening. I’d say as you downsize groups of people the better a conversation becomes as their is more focus and detracts it from being a group discussion in which points will be missed. Can you call an inner monologue a conversation? I don’t think so as you don’t really have an audience nor would you be engaging someone else. However with various recording devices and editing programs very much affordable these days you could quite easily record a conversation with yourself in both protagonist roles. Record a question or statement play it back and then record your answer to it… The point of this? Well, it could be most useful in an academic sense in a way to stimulate the cognitive process in essay writing, a part of University life which I always found annoying when working at home.
Ideal number of people for a conversation? I’d say a minimum of one good listener and perhaps no more then five.