I count 1 chicken per 2 people. AKA 20 chickens. ((In our area, pig would be cheaper... since chickens are $5-$9 each that's $100-$200 in poultry))
2 options:
1) Butterfly them. Cut out the spine with kitchen shears, season them up, lay out flat on tinfoil and loosely wrap each in the tinfoil (to allow the smoke t get in) or tinfoil pouch and get them going on the grill early in the day OR the day before. You'll probably need 3-4 rounds depending on how big your grill is.
AFTER the chickens are done use some big foil boxes to grill up any veggies/ sides. Set those aside and cover when done.
RIGHT before the party on a warm grill toss some wood chips for smoke and then pull them out of the pouches and "stack" or "domino" the bodies across. For some added lovely smoke/ to heat them all up again (but not cook them)/ keep them warm.
Have a cutting board beside the grill and quarter the chickens as people line up for them.
Most people will have 1-2 quarters over the course of the party. You'll PROBABLY end up with 5-10 chickens left over if you don't have reeeeally big eaters.
2) Cut them all up into pieces and grill individually (a pretty big pain turning 80 legs, 80 breasts, 80 thighs, etc.) ahead of the party. Have big serving dishes for each 'part'
Freeze a few quarters for meals later on. Shred some of the thighs (for tamales/ enchiladas/ etc). Cut the breasts into strips (for salads, quesadillas, bbq chicken pizza, etc.). Freeze the legs for kid snacks on the go. Separate out the wings and save for Buffalo Chicken wings (franks is the best buffalo sauce, imho).