PetExec will help to manage your pet counts by error checking the number of available pets selected to the # of pets entered within the boarding services. If you are scheduling for multiple pets and they are selected under available pets, then the # of pets field needs to match the number of available pets selected. This will pass that check and allow you to schedule. The # of pets field does not affect the pricing or the room reservations for the boarding. This check ensures that the number of pets boarding on the boarding count report is accurate.
If you prefer, you can go to Application Preferences-->Boarding-->Disable Boarding Pets Error checking. Check this preference to disable the error checking.