I flatted twice on a ride last week. But this was due to an old rim that finally gave up the ghost and cracked at the valve hole, so it wouldn't matter how many tubes I brought...

I usually bring a single tube and CO2 inflator with an extra cart, but it depends a bit on where I'm going and whether I'm alone. If I flat I usually stop at the next bike store I pass to get another tube, along with a CO2 cart and continue on. So it's more like needing two flats between bike stores... and there are a lot of those in Marin where I usually ride. There's even a 24x7 vending machine with velo necessities behind Mike's Bikes in Sausalito.

But if I go alone out someplace where I don't trust phone service and there's no population for miles I'll strap a Road Morph to the seat tube and stick a patch kit in the bag. Two flats is rare, but I'd rather be prepared for it when the alternative is to sit in the dirt in 100 degrees in the shade and no shade for hours. Or walk 30 miles on hot tarmac in socks or barefoot.