I've heard alot of people ask this question, so I decided to reasearch this.. Here is a chart:
*Please keep in mind that these are just averages, the amount of shots you get from your tank may vary due to inline pressure, and overall gun efficiency.
HERE IS A GUIDE FOR CO2 SHOTS PER OZ. AT 80º F.
65 TO 70 SHOTS PER OUNCE
7oz TANK = 466 SHOTS
9oz TANK = 600 SHOTS
12oz TANK = 800 SHOTS
20oz TANK = 1333 SHOTS
HERE IS A GUIDE FOR HPA SHOTS PER CUBIC INCH.
10 SHOTS PER ci. 15 SHOTS PER ci.
47ci = 470 SHOTS 68ci = 1020 SHOTS
68ci = 680 SHOTS 88ci = 1320 SHOTS
88ci = 880 SHOTS 114ci = 1720 SHOTS
114ci = 1140 SHOTS
--ripped from: http://www.geocities.com/divadcom/SHOT_CHART.html