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 “Sign up” (none of the other options have a capacity to set). You can also write a “Capacity Prompt” to inform Calendar visitors when the Event is full. Select your choice of capacity visibility for your Calendar.