Limit the number of group to 1

Hello,
I need to create 1 event with a capacity 20 registrants maximum.
I need to have only group registration
7 is the minimum number of registrants by group

My problem is : i want only 1 group maximum

My case is about children's birthdays
There is an event for a group of children (from 7 to 20 children)
If a group joins this event, another group can not participate.

By limiting the number of groups, here is the type of event that we could manage.