How many kegs of beer can you dispense with a CO2 tank?

With all the variable taken into consideration, below is a chart to help give you a rough estimate on how many kegs you will be able to dispense depending on your size CO2 tank.  If you are buying a prefilled and carbonated keg from a sotre or distributor, your will use about half as much CO2 to dispense that keg VS. the home brewer who is kegging flat coffee, force carbonating and then dispensing.  The process of force carbonating uses a lot of CO2 and takes time for the CO2 to absorbe into the liquid.  Once fully carbonated a consistent lower pressure is all that is needed to dispense and maintain a good carbonation level. 

A rough estimate to start from would be, for every half pound of CO2, you can dispense a will dispense a full size (1/2BBL / 15.5G) keg.  Which would also equate to about 3 - 5 Gallon kegs. 

If you are force carbonating you would use about double the amount of CO2 to force carbonate and then dispense the keg – 1 pound of CO2 to force carbonate and dispense a full size (1/2BBL / 15.5G) keg.  

Below the chart will help you determine the size CO2 tank that might be appropriate for your application.  When in doubt, or hosting an event, it's always better to habe more than less and run out and not be able to serve your beer, so buy the next size up tank to be safe. 

NOTE: This is just an estimate, as mentioned above many variable can effect these numbers.