How much water should a person have in a day?

Water: Six to eight glasses is recommended.

Depends...: The typical answer you will hear is drink 8 8-oz glasses of water/day. A better answer is drink at least 8 8-oz glasses of liquid/day. Water is by far the best choice. Other liquids (juices, soda, etc) are empty calories, and often a bad choice for fluid intake. Women typically need about 9 cups/day of water, and men about 13 cups/day... This will also vary with activity level (exercise).

Varies very widely: Like most issues, varies widely, most information is marketing; not science. With good health & comfortable environment, most people can get by, indefinitely, on as little as 0.5 liter/day up to as much as 25 liters/day, a 50 fold range, assuming good heart, liver, kidney + brain control functions. Different people in different situations choose different set points. As long as it works, ok.