Okay, let me do the math. Say the brisket flats you are getting are 3 lb each. 6 x 3 = 18 lbs of brisket. Plenty for 40-50 people. Word to the wise, wrap them at the stall. Folks at a big party like that won't care about the bark and small flats can be shoe leather if under or over cooked. Use the probe test to determine when done and don't be surprised if they finish at different times.
Racks of ribs? 6 racks x 12 bones = 72 bones divided by 50 = 1.5 bones (rounding up) per person.
9 racks x 12 bones = 108 bones divided by 50 = 2.2 bones (rounding up) per person.
I don't bother trimming spare ribs to the St. Louis cut for large pot luck parties (I actually don't bother trimming them at all any longer, but that's just me). The guys usually grab the larger bones. I've only had one person complain about the knuckles. You can easily do 6 or 7 racks if untrimmed.