CO2 is 44 g/mol, so 5 lb of CO2 is 51.5 mol. At STP, that occupies 305 gallons. So 5 lb could carb completely 152 gallons of completely flat beer to 2.0 volumes of CO2. However, beer coming out of a fermenter will have about 1.0 volumes of residual CO2 in it, so it would typically take 5 gallons, or 1.3 oz, of CO2 to carb a keg to 2.0 volumes (i.e. to add 1.0 volumes). Of course, you've got to push the beer out of the keg, and you probably flush the keg with CO2 as well before you fill it.
To look at it from the other angle, it will take 5 gallons of CO2 at STP to carb the keg from 1.0 to 2.0 vols, and another 8.3 gallons of CO2 to push that beer out of the keg at 10 psi serving pressure. If you flush the keg before filling, that's another maybe 10 gal of CO2. So each keg should use about 25 gal of CO2, or 6.5 oz.
Personally, I naturally carb in the keg (takes about 2 weeks at my cellar temps, which is about the same as set and forget force carbing, but doesn't take up kegerator space), which maybe also helps eliminate oxygen from the keg as well as saving a bit of CO2.
