"How much milk do we have?"
Translation:Ne kadar sütümüz var?
The way I learned it, if you own an indefinite thing, you use a construction like "my ... exists": kedim var "I have a cat" (literally, "my cat exists").
If you own a definite thing, e.g. you have been talking about a certain book and someone asks who has it at the moment, you use a contruction like "... is at me": kitap bende "I have the book" (literally, "the book is at me").
Bizde ne kadar süt var? uses something like the second construction - literally, something like "How much milk exists at us?" for "How much milk do we have (with us)?"
I believe that such -de constructions can also be used for when you have something with you even if it's not your own, e.g. "I have your book" would be Senin kitabın bende - you don't own the book but you have it at the moment.
Or Şemsiyen var mı? would be "Do you have an umbrella?" in the sense of "do you own one?" even if you forgot it at home, while Sende şemsiye var mı? would be "Do you have an umbrella (on you/with you)?"