A keg in the US is a 1/2 barrel keg. If a barrel of beer is 31 gallons, a keg would have 15 1/2 gallons (1,984 ounces) of beer and a pony keg (1/4 barrel) holds 7.5 gallons (960 ounces) of beer.
If you are serving the beer in twelve ounce cups, that would equal roughly 165 cups for a keg and 80 cups for a pony keg. The general rule of thumb for serving alcoholic beverages is to expect your guests to consume 2 drinks the first hour of the party and 1 drink every additional hour.
This calculator assumes that beer will be the primary alcohol served. If you are serving other drinks/cocktails, you would need 1/3 of that amount.