I carry 3 or 4 cartridges. If I need more than that the cycling gods are against me and I'm calling in the team car(wife) for ride. On a group ride everybody has co2 because it would be inconsiderate to make everbody wait for you to pump. It also means if you run out of cartridges there's always somebody there to offer one up. I bought a couple of boxes when I first got my bike 2 years ago and still half of those so the cost is minimal unless you're having a string of bad luck.