The number is zero gallons, or sometimes very little.
Trying to put out the self-oxidizing reaction of a lithium battery that way is a waste of time and water. Fire fighting teams that try to put it out like that will see the flames go down, then they let off, and then it starts right up again. In the end, the car burns through the entire battery pack one way or another. You’re just delaying the inevitable, and it takes most of the day and swimming pools of water to get done.
Fire crews need new training and equipment. If possible, the car needs to be towed to a safer location. Then you let it burn. There are fire proof blankets that can cover the car to prevent spreading the fire elsewhere. Takes about an hour. Sometimes, a little water helps prevent spreading to other things, but it’s minimal.