How many gallons of water does it take to make one glass of milk?

2,000 gallons.

It can take up to 2,000 gallons of water to produce one gallon of milk. The cow needs water to perform basic biological functions from day to day, and only a fraction of the water the cow consumes is actually converted into milk. The fact that it takes so much water to produce cow's milk means that anytime you or any consumer chooses to drink milk, the burden you place on the natural environment is a thousand times greater than if you were to consume water itself. Drinking one gallon of milk is like pouring 1,999 gallons of fresh water down the drain.

http://www.naturalnews.com/023341.html

Hmmmm....ok, the question says one glass of milk.
My source bases the 2,000 gallons on one gallon of milk.
One gallon = 128 ounces/8 ounces in one glass = 16 glasses
2,000 / 16 glasses = 125 gallons of water per glass of milk

It's not 125 gallons of water per glass of milk?

This site provides an even lower number, 49 gallons per glass of milk.

It takes almost 49 gallons of water to produce just one eight-ounce glass of milk. That includes water consumed by the cow and to grow the food she eats, plus water used to process the milk.

https://www.greenandsave.com/greenoffice/faq