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. 

If it shows canine/feline, DO NOT delete the pet type.  Instead, click on each one and rename it to cat or dog as needed.  Hit Save when done.