Two half liter bottles are enough for me. Very hot days I use effervescent (do you call them effervescent?) tablets against dehydration. Along the camino I fill up my bottles in the fuentes, bars and restaurants as well as I drink some extra in the morning when having breakfast, also for lunch and keep on drinking all evening, too. This way 1 L of water is enough to carry, even the hottest day. Don´t want to carry more water than necessary.
Water is very available along the camino, as well as other drinks like Coca Cola, other soft drinks or beer. Once when it was HOT HOT somewhere before Pamplona and Trinidad de Arre, in a little one street village there happened to be a Coca Cola machine just when we were just wanting to drink something very cold. I don´t drink Coca Cola otherwise but IT WAS GREAT in the heat of camino! annie