Depends On Level of Drinking
It depends on how long your party is. Based on a three-hour party, purchase 100 beers for a crowd of light drinkers (two beers per person), purchase 200 beers for average drinkers (four beers per person) and 300 beers for a crowd of heavy drinkers (six beers per person). A keg of beer (or a half-barrel of beer) generally holds 124 pints of beer or the equivalent of 165 bottles, and would be cheaper than buying the equivalent in cases. One keg would be more than enough for a party of light drinkers and two kegs would suffice for a party of heavy drinkers, taking into consideration that some volume would fizz away as part of the suds.
Not Everyone Likes Beer
According to NB Liquor and That's the Spirit, on average between 50 and 60 percent of your party guests will consume beer. The others will consume spirits, wine or non-alcoholic beverages. Thus, if you are purchasing beer for a three-hour party for a crowd of average drinkers, you would only need between 100 and 120 beers.
Bottom Line
Take into consideration the length of the party and the guests you are inviting. Think about who drinks beer and the average level of drinking of your guests. If guests are planning on driving home, keep the level of drinking to a minimum. If your party is too small to merit a bartender, ask several responsible friends to serve drinks, so they can keep an eye on your guests' levels of sobriety. Serve snacks, juice and water throughout the night to reduce blood-alcohol levels. The Queensland Police recommends that hosts stop serving alcohol one hour before the party ends. A total of one keg of beer for 50 guests would also keep drinking to a minimum.