How many 16oz bottles of water should a 20 year old drink a day?

Unsure: What i tell everybody is that we are all different. The test for me is to look at the urine upon urination. If the urine is clear like water you are likely drinking plenty. If yellow you are dehydrated.

Doesn't matter: They should take in enough fluids daily to pee every 6-8 hours. Counting ounces is best left to OCD people with no life. You get water value from many foods (applesauce 95%, vegetables 60%) and fluids (soups, soda,juice). You could live your life and never drink water. If they are in good health, imposing such a goal or requirement is unnecessary.