COYO creates a notification if users join a community or leave it. These are received by both users and admins in the community.
COYO totals up the members in these notifications. You thus receive notifications like these:
This does not mean that 8 new members have joined the community, but that (here) Verena Meier has joined after 7 members were already in the community.
Things are somewhat different when more than 50 members belong to the community. Then only the admins receive the notification, and no longer the users. This mechanism prevents users from receiving too many notifications.
These rules cannot be customized.