scully, we serve mimosas at our annual New Year's Day party to everyone who walks in the door who didn't get enough to drink the night before.
In one of the early years of this ritual, four of us killed 8 bottles of champagne. We like to drink and play loud music. This year, we had leftover bottles. Apparently we run with a soberer crowd these days (plus the kids are older now and notice things).
If you are all champion partyers, I recommend a two-person-per bottle ratio as a starting point. Two to three gallons of OJ (pulp-free, please) should be plenty, unless you're serving mundane bubbly and want to mask the taste.
Kool Aid - Think before you drink.