你好，你想学习中文吗？

#1 is wrong but you can say it like this: "We have food for how many days?" Simply putting the question mark at the end turns it into a question. When you speak it, you need to add rising interrogative inflection at the end of the sentence.

#2 is fine, as Dan said. To make it sound even more natural, I would suggest: How long will our food last?

#2 is correct and is good, natural English. #3 can be made correct by adding the word "for:" "For how many days will we have enough food?" (You can also say "How many days will we have enough food for?" This is correct and natural, but it poses a strange problem: there is a grammar myth that says it is "bad English" to end a sentence with a preposition.) #1 is not correct and cannot easily be corrected. You can ask "Do we have enough food for five days?" but you can't substitute "how many" for "five."

I think that yes. But I am not a teacher.