There is no right size for a support group. Some big patient support groups may have thousands of members. A local group for a single issue may be very small.
The basic decision of building your group is thinking through your capacity, versus your ambition. Growing too quickly, like a tree without strong roots, can cause a lot of difficulties. For example, 100 people connect with you but you only manage to respond to 20.
You also need to think about how you measure how you are going, and one of the most important group objectives needs to be about people feeling better, rather than doing more. The evidence base is very clear that support groups can really help people, the challenge is to make sure that you are!