I would do at least 40lbs of raw weight, that should yield around 20lbs cooked or .6 lbs per person. I recently did 16lbs raw weight for a party of 20, men and women 18-50 years of age. Some were big eaters, others small, some chose to drink their meal as the booze was flowing. There were enough leftovers for 2 sammies. Sides included beans, potato salad, mac and cheese, and a pasta salad.
Dedicated Stick Burner, Pitmaster @ All About The Q, facebook.com/itsallabouttheq KCBS CBJ