Ok how many shots you get off air tanks depends not only on the size but the pressure of the gas inside.
So you'll get more shots out of a 4500psi 68cu tank than a 3000psi 68 cu tank. There is a rough chart that ive found on the net telling you (on average) how much you should get out of each tank.
3000psi
10 SHOTS PER ci.
47ci = 470 SHOTS
68ci = 680 SHOTS
88ci = 880 SHOTS
114ci = 1140 SHOTS
4500psi
15 SHOTS PER ci.
68ci = 1020 SHOTS
88ci = 1320 SHOTS
114ci = 1720 SHOTS