Originally Posted by nondes

Considering how much folks spend on lighter bike components, there's no sense in putting it all back on in water bottles if you don't have to. Considerations for size/number of bottles include weather (temperature and humidity), length and difficulty of ride, availability of rest stops on an organised ride or convenience stores/water fountains, whether you want a separate supply of water vs. energy drink, and bike aesthetics if you think two bottles looks a bit clunky or if you might need to add a non-matching bottle holder for a longer trip. Remember you can take in a lot of your fluids at rest stops rather than while riding.