I have a vertical propane smoker that has slots for 7 racks. I will be smoking 6 racks of baby back ribs. I have smoked 3 racks (9 pounds) at 200-210 in six hours. I know I will need additional time for the extra 3 racks -- but it can't be 6 extra hours (12 total). What is a good rule of thumb?
The extra cooking time would be negligible. Cooking time is predominantly a function of temperature of the cooker, and surface area and mass of what you are cooking.