Humans use more fresh water than just the quantity that they actually drink. You also have to include the quantity used for sanitation, personal hygiene, and recreation. Even discounting non-household use, a single person can easily clear 100 L (0.1 m^3) per day through their water meter, without even trying.

Go read your water bill some time. Divide your metered usage by 30.5 and restart your calculation on a new bar napkin or envelope. That gives you a multiple of about 26, if the whole planet consumes water like American households, and agricultural and recreational uses are completely ignored. Use that same 10% figure, and the whole Amazon could be completely consumed by only 18.2 billion people.

That's just one river, but it is the one with the most water in it.