I'm guessing that 120 miles a week tranlsates into something between 20 miles a ride, 6 days a week, or 40 miles 3 times a week. So if your typical ride is between 20-40 miles (as is the case with many cyclists), you're theoretically never more than 20 miles away from home in an out-and-back route (shorter if you have more of a loop).

Assuming that you have a cell phone and somebody to come to your rescue in extreme situations, you really don't need to prepare like you're crossing the siberian tundra on your bike.

IMHO, if you flat more than twice, then there's probably something wrong that you may or may not be able to correct on the road assuming it's the same tire. Maybe you missed a piece of glass in the tire the first time, but if you miss it the 2nd time, you probably won't have much luck figuring it out without spending some time figuring out what's going on.

So if you're still buying into my assumptions which are based on my own experience, I carry one spare tube, a patch kit, and three CO2 cartridges. As long as I'm not intentionally riding through glass, I've never needed more than one cartridge to get home. Having 2 extras is a good safety margin as far as I'm concerned.

I also figure that if I'm out on a 2-3 hour ride (<50 miles or so, max of 25 miles from home) I'll probably plenty pissed off if I've flatted 3 times in that amount of time, and will be more than happy to call the cavalry for a ride home (or just take the $20 in my seat bag to the nearest bar to wait for a ride).

Anyway, if you get the CO2 thing, buy extra cartridges right up front and practice using them a couple of times at home. It's not difficult, but it's probably less stressful to figure out in the comfort of your own home than on the side of the road in a rainstorm.