A 14lb (raw weight) packer brisket should yield around 14-8oz servings (50% loss in cook). A 8.5 lb (raw weight) butt should yield around 8-8oz servings (50% loss in cook).
If you plan for a 50/50 ratio of pork/brisket for 75 people you would need a minimum of 3 packers and 5 butts. If you are guessing that some will substitute ribs for pork and/or brisket then you could cut back accordingly on the number of packers/butts.
[raw weight (lbs) of packer/butt]*0.5 (cook loss)*16oz/[oz serving size]
14lb*0.5=7lb, 7lb*16oz=112oz, 112oz/8oz serving size=14 servings
8.5lb*0.5=4.25lb, 4.25lb*16oz=68oz, 68oz/8oz serving size=8.5 servings
Brisket: 75 guests*0.5=37.5 servings, 37.7/14 servings per packer=2.7 packers
Pork: 75 guests*0.5=37.5 servings, 37.7/8.5 servings per packer=4.4 packers
Do I smell SMOKE?????