How many 5 gallon kegs will a 20oz CO2 tank carbonate?
There are a few factors that come into play here and depending on if you are using your CO2 to force carbonate and dispense, or strictly dispense an already carbonated keg.
This answer all depends on how much pressure you are putting into your kegs and how high you are force carbonating at. In short, you should be able to get atleast a few (4-5) kegs carbonated and served off of a 20oz CO2 tank.
If you are using the 20oz CO2 tank strickly for serving already carbonated kegs, it will last you FAR beyond that!