It all depends on the frequency of workouts and daily activity. 2 litres is the recognized quantity of water to be taken in for the average medium-active person, but someone that works out daily for example, they can expect to at least double that to 4 litres.
Personally i would struggle to take in more that that. If you're wanting to lose weight, then CV is probably frequent in your workouts, so 6 litres may be needed. it has to be remembered that constant intake of large amounts of fluid can often dilute the minerals etc from your blood and literally flush out some of the goodness that you have taken in during the day.