If you are restricted on the number of pets you can have onsite at once, in KennelBooker you can limit the number of pets that can be onsite at once from the Boarding settings tab in the settings area.
On the settings page you can enable the capacity checking by clicking on the checkbox and setting the total number of pets you can have onsite into the provided checkbox.
You can also include daycare pets in this count as well if required.
You can also choose to include boarding pets that are checking out on a particular day in the count.
When you go to add a booking the system will check you capacity first, if you are going to go over your limit the system will alert you, tell you what days you will be over on, allow you to override the limits or move the booking to a waiting list instead!