Once they hit the ground, it is not advisable to pick them up and use them in your gun. The more you buy, the better the price. You probably won't shoot it all in one day unless you split up a case with others in your group. So you can get maybe 5 trips out of one 2500 shot case on your own. The tanks also needs to be refilled with CO2.
Some guns also use compressed air.
A 9 oz tank will usually be good for 300 to 400 shots depending on the gun. Larger tanks might be good for 2 or 3 average days of play. I mentioned the mask. This is a minimum requirement for safety. A thermal lens has the ability to prevent fogging up which is obviously a tremendous advantage.