I think as a general rule, you should drink about 1L/hour at temperatures below 25℃ (make that 1qt at temps below 77℉), more at higher temperatures, most bike bottles are 750mm (¾L), so for rides shorter then 45 minutes at cooler temperatures, one bottle is sufficient. Make it warmer, or the ride longer, and you need more. Realise that water, bought in disposable bottles in stores, costs about twice the price of gas, and means all that plastic to dispose of. There is a bigger issue though, and that is, if you don't carry enough, you can run out, in the heat of a ride, you may not want to stop, you get dehydrated and end up with a much bigger performance issue then the kilo and a half for a couple of extra bottles. I have 2 bottle mounts, and will often carry extra, but usually in a pannier, along with a couple of kilograms worth of camera equipment, food, extra clothing, etc. The bike is a converted mountain bike, it weighs 15kg, rigged out for riding distance, it's often over 20kg. I know for a lot of roadies where bikes of 9kg are considered heavy, that's a lot, but I would rather carry something and not need it, then not carry it, and need it.