I carry two 800ml bottles on the frame.
I've gotten used to carrying one in the jersey pocket when needed. Usually a gatorade/etc so I can ditch it when empty. Works well with tight jerseys but causes loose jerseys to hang somewhat. This will do me for a fast no stop century or a slower 200k, unless the heat gets silly.
I often head off with just one bottle filled and powder for two bottles in the other, theres usually a school 30-60km away to fill up at. A small can of energy drink or coffee in the seat or handlebar bag is a nice pick me up.