"How much water do you drink?"
Translation:तुम कितना पानी पीती हो?
No, it should be "tum kitna paani peeti ho? " OR "kitna paani peeti ho tum?! "
The first one means:- he's asking, "HOW MUCH WATER DO YOU(usually ) DRINK?"
The second one:- it might mean the same as first one,
It can also be an expression like, "HOW MUCH WATER CAN YOU DRINK?!!" Maybe she's drinking a lot and he's surprised by the amount
But both can be correct here depending on the context but while you speak, your tone may differ, so first first one fits more.
Hope this helps, sorry for my bad english
I did some internet research and found this one: https://www.successcds.net/learn-english/countable-and-uncountable-nouns-difference-examples-exercises.html
Search for "6. Uncountable noun" - and you find that they have to be treated as plural. "कितने चावल बचे हैं?" is given as an example there.
Keep in mind that this site (in the link you gave) is not meant for learning Hindi, it's tailored for Hindi speakers to learn English. I guess in Hindi चावल is treated as a countable noun, more akin to "grains of rice," however for uncountable nouns you definitely don't use the plural, you just use singular. Otherwise you're saying something like "How many water" which just doesn't make sense, it's कितना.