As a general rule of thumb, it takes about a ½ Lb of CO2 to dispense a ¼ barrel of draft beer and 1 Lb of CO2to dispense a ½ barrel of draft beer. This table lists the approximate number of kegs that can be dispensed from each size of CO2 cylinder.


 

 

2-1/2 Pound Cylinder

5 Pound Cylinder

10 Pound Cylinder

15 Pound Cylinder

20 Pound Cylinder

Home Brew (5.00 Gallon)

7-11

15-22

31-44

46-66

62-87

Sixth Barrel (5.23 Gallon)

7-11

14-21

29-42

44-63

59-83

Quarter Barrel (7.75 Gallon)

5-7

10-14

20-28

30-42

40-56

Half Barrel (15.50 Gallon)

2-4

5-7

10-14

15-21

20-28


 
A gas tank that is properly filled should read about 750 PSIG when the cylinder is at a 72°F temperature. CO2 gas pressure increases when it is exposed to heat, so if the gas tank temperature is above 72°F, the pressure will read higher than 750 PSIG. If the gas tank temperature is colder then 72°F, the pressure will read lower then 750 PSIG.

The tank pressure should read 750 PSIG until the liquid CO2 in the cylinder is gone, and the tank is almost empty. A decreasing pressure indicates the cylinder is nearly empty.
 
Ref: http://www.micromatic.com/beer-questions/how-many-beer-kegs-dispensed-out-co2-tank-aid-89.html