Give a lot of consideration to a Camelback or similar backpack-type water reservoir. Even the small ones hold the equivalent of two water bottles and the big ones up to four. They also make drinking more convenient and thus less likely to be ignored until you are dehydrated. I have used one on long rides and was very happy to have it.
+1. I use two bottles plus a small CamelBack. 40 oz. I believe. For my first tour I didn't use one and instead took three standard bottles. That often proved not enough, especially in hot, humid weather. My second tour was in southern Spain. I ended up buying a small CamelBack about a week into the tour because it was warm and there were often long distances between services. I thought I would hate having it on my back, but it did not bother me a bit. They are also handy in camp if you are not close to a water source. I think the big ones now go up to 100 oz., if not more.