The LP5 uses the gas to re-cock the bolt as well as shoot the pellets. It is in effect a blowback pistol so will have a much reduced shot count than a single shot. Having said that I don't seem to have to refill my cylinders that often.
Baz forgot to mention that to get a good fill in a Co2 cylinder you will need to put the pistol cylinder in the freezer for about half an hour and it can help to put the Fire extinguisher / Co2 bottle somewhere warm during this time.