One 16 gram cartridge will fill up a tire no problem. I carry three cartridges and if I were to ride a century, I would carry four. I have never had four flats in a day, I have never even had two flats in a day.

After you use CO2, it will be hard to go back to a pump. Unless you are the type that wouldn't mind giving up indoor plumbing for that long walk through the snow to the out house.