When heading out for a long summertime ride, you need plenty of water. How much depends on your size and the weather conditions, and probably on various individual differences in how much you sweat, how well hydrated you were to begin with and who knows what else. Here’s one article on the topic, there are plenty of others. Of course the information they provide varies so in the end you have to figure it out for yourself, using this information as a guide.
My bike frame is small and had room for only one bottle cage. This isn’t enough in the summer. I’ve tried hydration packs. They let you carry a lot of water and allow you to sip at will. But after trying various models and styles I have come to the conclusion I just hate them. I don’t like how they feel on my body and they trap sweat on my back, increasing discomfort greatly. While at Race Pace recently I found a bottle cage that mounts on the handlebars. It’s not pretty but it works. I don’t notice any change in handling having a water bottle on the handlebars.
If I’m out in very hot weather for more than a few hours, I have to refill or carry one more bottle in my trunk sack. Most of the time two large bottles is enough. I’m happy to have found this solution.