I just picked up a 16 grm CO2, that'll get a road tire up to 125/135 no problem, or so they say. With the 12 you can get 90 #s of pressure in there, and at least make home to top it off, so I usually carry 2. I learned a tough lesson, wasting one and then getting stuck when it didn't fill the tire. So, now I have two.
Oh, and if you are carrying them don't have the valve screwed on, turn the cartridge around backwards that way you won't discharge the cartridge accidentaly.