this topic needs to be a sticky...... I think it is asked weekly.
Seems the general concensus is 12-16oz to forcecarb and dispense 1x 5gal corny.
so, 5-8 kegs is a reasonable number. It also has other variables. How much gas you use to push cleaner and sterilizers, and stuff.
I am on my 3rd, and I expect to refill after 5. I also use my tank to push cleaner and sanitizer through my lines, which will eat it up a little quicker.
If i get a good deal on a 20lb, I would measure for room to fit it in my keezer