A typical beer is 5 grams/liter carbonation, so about 90 grams CO2 per 5 gallon for carbonation.
CO2 at 50F and 10psi is about 25 gram/5 gallons (.01903m3 * 68947N/m2 / 188.9J/kgK / 283K=.0245kg)
So, including tubing, say 150 grams/5 gallon batch, a 5 lb tank should be able to carbonate and push out 15 5-gallon Corney kegs...
I use CO2 for all sorts of things (clearing headspace, Beergun, starting a transfer) but even when I am just using it to carbonate and serve I typically feel like 6-8 kegs is a more realistic number. Unnoticed leaks have brought me to the liquor store twice in two consecutive days more than once. Also, beverage tubing is going to have some level of permeability as are quick disconnects and other fittings.