If you have a drop location that has a strict threshold for the number of sign-ups, go to Deliveries/Drop Off Sites, choose the location you are trying to change, scroll to the 'Maximum Subs' row, enter in the maximum amount of subscriptions allowed, and check the 'Enforce Maximum Subscriptions' box. Once that location reaches that number, the software will no longer allow new sign-ups.
If the situation changes, i.e. space is available for more boxes, uncheck the Enforce Maximum Subscriptions box, Save Changes.
This is also useful to accommodate the physical space limitations of a truck.