Answer: A lot, but depending of how much pasta/rice you´re cooking.
Explanation: There are several users of the pasta method around the Internet that define the correct amount of water with "what seems right to you", and believe it or not this migh not be a lie at all. One of the constants that I've found on this method is the instinct. Put as much water as you wish, but your boiling vessel has to look good! A recipe that generates crave at sight.