You wont find a solid answer. It's like asking how many miles per gallon your car gets and not taking into account different makes and models. Things that will off set the # of kegs are leaks, Vols, CO2 used to purge normal air to reduce oxidation, how you carb and so on. All in all the gas is cheap and the bigger the tank the cheaper it is. My 5# is $20 to fill but by 20# is $24.
The number I found is any where from 10 kegs up to 44 5 gal kegs. You wont really know till you have you operation set up and running.