I get 4 brews. But. There are lots of ways to heat, and people do it differently, even with 5 gallon batches.
1. I heat my preheat water in the house, where the propane is MUCH cheaper than in 20# cylinders. The preheat water later becomes my sparge water.
2. I heat the strike water on my propane burner in the garage- but I'm using a fairly small (for 5 gal. AG) brewpot, only 30 qt., AND it's insulated (I used automotive firewall insulation from JC Whitney).
3. For the boil, I do not attempt to get the "volcanic" boil that is seen on many videos. I just shoot for a nice barely rolling boil, and leave the gas set there.
So- I get 4 batches, but someone who does things differently is likely to get different mileage from a cylinder.