If you are serving ribs and chicken, I seriously doubt you will run out of food by cooking 9 racks of ribs figuring 2 per person unless you are serving 50 men. At most 10 racks. Most ladies will not take more than 1 rib if they take chicken too.
Also, the last time I thought there were 3 racks in one cryovac, there ended up being 2. I now buy my ribs from Sysco and they come 12 racks in a case. I only buy St Louis style as I know I will always get 12 bones in a rack.
I would cook 50 chicken quarters (mixed dark and white) and 9/10 racks of ribs. You are also serving 3 sides plus desserts.