What group size is ideal for group therapy?

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.