It depends on a few factors.

If you are using the CO2 only to dispense the beer, not to carbonate it, you may get as many as thirty 5 gallon Cornelius kegs out of one 5 lb. CO2 cylinder.

However, most homebrewers are also using CO2 to force carbonate their brew, so this brings it down to around ten 5 gallon Cornelius kegs out of one 5 lb. CO2 cylinder.

Your results may vary, depending on your desired level of carbonation, the length of time the keg is kept under pressure, the length of your gas and beer lines, and many other factors. If you are not getting anywhere near these numbers, check your keg for leakage.