There is no hard and fast rule. It depends on the topic, instructor, and participants.
Nevertheless, the larger the group, the more impersonal it may feel. Having a group size of 4-6 people makes it easier for the group to develop a stronger bond.
However, even a group of 15-20 people can develop a bond remotely, it just may take more time and effort to do so. One advantage of a larger group is that an absence of one member may not have as much of an impact on the discussion.