This gets discussed a lot in the forums. The general rule is to take the heaviest piece and use it's weight in calculating an approximate cooking time. For example, if I want to cook a 5lb pork butt for 1.5 hrs per pound at 235 degrees, the cook time would be approximately 7.5 hours.
However, when we have additional pieces of meat to consider, the cooking time will be longer. Heat from the fire is energy. Each piece of meat has to absorb energy in order to cook and reach it's desired temp of 190-205 degrees. The method above doesn't take that into consideration. It assumes that all pieces will absorb the same amount of energy evenly.
Anyone that has watched meat plateau for an hour or two knows better. I suspect that the approximate cooking time for 8 pieces of butt at 5 lbs each will be approximately 12 hours. Thats 4.5 hours more then the standard formula suggests.
No matter what formula you use, you are going to cook to temperature, so have a good thermometer and start checking your smallest pieces after about 8 hours. When they hit 190, take em out and wrap in foil to set up.
I would love to see your smoker full of pork. Qview it please.