I like to mount 3 bottle holders on all my bikes. 1 for water ~ 1 for tool container ~ 1 for headlight battery

In the summer when I ride 40+ miles I carry 2 water bottles but often barely need the 2nd cuz I find liquids along my route. If you'll be ridign through towns and stopping for lunch, etc, then 1 water bottle is enough. If you're carrying food and may not be in towns with stores, then bring 2 water bottles and sprinkle a little salt into each. don't forget your potassium and a tiny bit of magnesium.