Two (or more than two). Means you can always keep one with pure water (to pour over head, wash out wounds, clean hands) and one with something more useful for hot riding days (including fruit juice, and perhaps a little salt and sugar if you are not in Gatorade territory). Also the plastic off the shelf bottles break, crack and otherwise get destroyed, while you can always use the sigg ones as hot water bottles if it's cold at night.
__________________
Zero gallons to the mile