There are a lot of differing answers to that question, but here's one version:
“It depends on your size and weight, and also on your activity level and where you live,”“In general, you should try to drink between half an ounce and an ounce of water for each pound you weigh, every day.” For example, if you weigh 150 pounds, that would be 75 to 150 ounces of water a day.
Because singing tends to dry your throat and vocal cords out, due to almost continuous air passing over the vocal cords, you will need to do a good job of hydrating yourself.
I bring about a gallon and a half of water to gigs with me, and probably drink about half of that, sometimes more. On some occasions, I drink almost all of it. I'm certainly ready for break time after about an hour and a half of singing and playing. We have to stop or we'll have to mop.
If you just carry a bottle of water wherever you go, and take a swig when you think of it, you'll probably do OK. Alcoholic drinks do not hydrate you. They rob your cords of hydration.