3-5 liters in panniers, and as many on-the-bike bottles as it can manage. In the past, that's been one bottle. My touring bike now has braze-ons for three cages, and I fully intend to make use of all of them next time I go on tour. The last tour I went on, I carried three liters in my panniers in a 1 liter Nalgene bottle and four .5 liter plastic water bottles. My touring partner carried two 1 liter Nalgene bottles in his panniers, and each of us had a small bike bottle. Altogether, we had more than 5 liters between us, and it wasn't enough for two people. We were forced to refill from a river in the remoter part of north western Massachusetts. This is all the evidence I could ever need that carrying iodine tablets along, even on a road tour where you don't expect to need them, is cheap insurance! It took us some three hours or so before we got to a place where we could dump the nasty iodized water in favor of gas station bathroom tap water, so I'm very glad we had the stuff.

Boiling water with your camp stove isn't an option. We didn't realize at first that we had the iodine tablets (we had very cleverly stashed them with our first aid supplies, and brought them along by accident!), and we wasted half of our stove fuel boiling perhaps a liter and half of water.