For a good conversation I prefer to have a group of 4.  Four individuals is enough to add diversity and depth into any group dynamic, with story swapping, and funny interjections.  Any more than that and there is a risk of one voice being buried underneath all the others.  When you have less than that it seems that the conversation lends itself to a more intimate variety instead of lively or boisterous.  There are many types, but that is my favorite.