I carry 2 tubes, 2 cartridges, and both a pump and CO2 inflator, and a patch kit. Because you just never know what is going to work, or if you are going to run out of tubes on an especially bad day. Or in a fit of benovelence you give a tube or cartridge away to some other unprepared cyclist.

And for anyone who has not actually used the CO2 inflator- its a very good idea to sacrifice a cartridge to learn how to use it in the comfort of your living room. Otherwise you'll be likely be sacrificing that cartridge when you really really need it. When it's cold, or getting dark.