That's hard to answer you have to break it down a bit:
-1 gallon per day of water per person to drink

Then add the following:
- How many days are you preping for? 7 days min?
- Are you using water to cook with? add 1/2 gallon per day
- Are you using water to bath with? add 1 gallon per person per day

Total for 7 days: 17.5 gallons X 2 people = 35 gallons <img src="/images/graemlins/cool.gif" alt="" />

You can never have enough water in a survival situation. If you can store it you should.

Adam