Can't put a number on it. Like with the now popular caffeine drinks available, there are documented deaths contributed to both. Ones health status, underlying conditions and personal sensitivities to caffeine play a huge part in what is tolerable for each individual. So, while it may take a hundred cups for one, it might take only a couple or three cups for another to experience cardiac arrythemias from too much caffeine.
I can't drink a single cup of coffee. If I do, my heart races and I get the shakes... with one cup! I can't imagine what would happen if I drank an energy drink.
It's odd that you'd post this OP - I was just having this conversation/argument with someone on FB. Their cousin kept saying they needed to drink insane amounts of water on top of any other fluid they had. (soda, fruit juice, tea, coffee) I had to point out that everyone has a different limit for how much they needed to drink a day, which also depended on your activity that day, the weather, etc. It's called moderation. Anything above or below that will put you at risk of unhealthy side effects.. just like everything else in the world.