It happened to me and it’s not pleasant - you have to get that IV of electrolytes in there ASAP. Even so I was in the hospital for a couple of days.
The amount probably to some extent depends on your body size and weight, but you don’t really know your tolerance so a general guideline is good.
But the moral of the story is don’t drink plain water - always something with electrolytes or even a sweetened beverage.
I use Liquid IV powdered electrolytes to put into my water bottle when I’m doing outdoor activities. Highly recommended!
Use them after lifting or sprinting training.