Depends on the size of the keg (in liters) and the size of the glasses (often in liters or milliliters) used: simply divide the larger number by the smaller number (in the same unit) and you’ll get the approximate number of individual beers, not counting any spillage while connecting the keg to tap equipment and while tapping beers from it.
General rule of thumb shown below for full barrels. A standard keg is 1/2 barrel.
Why use one, 7, and 14 barrels, you ask? Because those are common batch sizes for small breweries/taphouses, so these are the starting points for calculating profitability.
1bbl = about 31 gallons, or 248 sixteen ounce pints
7bbl = 1736 pints
14bbl = 3472 pints
1 beer. You can only put 1 beer in 1 keg. If however, you want to blend beers and put it in that keg? Well, it is still 1 beer, though it be a blend of many.