It was based on a misunderstanding: People need the equivalent of 6 glasses of water a day, but a large chunk of that comes in through food and drink you already have. For most, adding one or two glasses of water a day would put you up at the goal.

That said, I don't recall where the target number came from.

They need no such thing. The target number (usually quoted as 8) is pure folk mythology, there never was a study that concluded that that was the right number for some medical/biological reason. This has been in the news.

The correct number depends on individual, weather, activity level, etc. The actual medical recommendation is merely that healthy people should pay attention to their thirst and drink when thirsty, and plan ahead for exertion and desert excursions etc.

Certainly some people don't pay attention to their own thirst, but that's not to say that 6 or 8 are the correct numbers. That may be too much or too little.