If you're camping by yourself then the math is simple for per person per day. 64 ounces (roughly 2 liters or a half a gallon) is the amount an average adult should drink a day while out camping and hiking. So, two 32 ounce Nalgene bottles a day should be your goal to drink. So, add that amount together for three days. It will total to approximately 6 liters of water which is 192 ounces, or 1.5 gallons. That's just drinking water. You'll need likely a gallon extra or more for washing up and cooking. The math will get a little more complicated when having to pack for multiple people, but the same principle applies. The number will just be significantly larger for the number of people and the number of days you'll be out there. This is why camping around water sources can be extremely convenient because you can utilize them and that means you don't have to carry as much water with you in your pack.