So this topic is actually one of my personal bugbears.
I used to buy into the whole "drink 8 glasses of water a day" thing and would dutifully bring a big bottle of water everywhere and chug it all the time. Especially since I used to live in Colorado where everyone tells you to "stay hydrated!" because of the altitude. Eventually I started noticing my mouth was feeling dry and I was getting headaches. I thought maybe I was dehydrated and started drinking even more water to compensate, but it just got worse.
Finally, only after many months, I happened to read online about somebody with the same problem, who realized they were actually overhydrating. Symptoms of which include... dry mouth and headaches. Sure enough, after I cut down on my water intake, the headaches and dry mouth went away.
Now I don't hydrate up to any particular target, I just drink when I'm thirsty.
The latest research indicates that people only really need 1.5-1.8L of water. It also depends on many factors like your age, weight, diet, how much you exercise, etc.
https://www.bbc.com/news/uk-scotland-north-east-orkney-shetland-63755475
So my advice to everyone would be: listen to your body and do what's right for you.
(It's probably also a good idea to replace sugary drinks with water, but that's kind of a separate topic!)