If you keep getting an overbooking message for boarding, make sure your settings are correct.
Click on File, System Options, Boarding tab.
If you have 'Require run numbers' unchecked, you will see boxes for Maximum dogs and Maximum cats. You must enter your numbers here. If you put 50 for maximum dogs, then KC will not warn you until you try to book dog #51.
If you are getting the overbooking message even though you have less appointments than your maximum numbers, make sure your pet types are set as Cat and Dog in File, Lists, Pet Types. KC does not recognize canine and feline.