Scoop: a keg -the ones we have at parties in trash cans full of ice- is 15.5 gallons. A pony keg is 7 gal, a soda keg is 5 gal. 128 ounces in a gallon. To figure out how many beers in a keg you also need to know how big of a cup you are using. Most party cups, like the good old red solo cup are 16 ounces, but 12 is also common. There are also 12 ounces in a regular can or bottle or beer. Screw it:
12 oz = 165.3 beers, almost 7 cases
16 oz = 124 beers/124 pints
Hope this answers your question. Sorry I rambled on so.