You can limit how many people sign up for a Group Sign Up event by setting a Booking Capacity for your Event.
To set a Booking Capacity, go to the “Self-Book” Dashboard, and click on the “Add Template” button. Then, under “Request Type,” select “Group Signup” (none of the other options have a capacity to set).
When your capacity has been met, you can also set up a waitlist and choose the capacity for that waitlist. Both your event and your waitlist can have a "Capacity Prompt" which is a message that visitors will see when both lists are full. You can choose to show the visibility of the capacity to your visitors or not. See the image below: