Suppose I have a friend. I know he has only one child.
If I ask my friend:
How many child/children do you have?
In your opinion,in this situation, “child” or “children” is correct? (I have already known that my friend has only one child but I still ask).
I have a question for you. If you know, as you say, that your friend has only one child, why do you ask how many children he has? !!!
I agree with Alan. Sometimes your questions really don’t make much sense. This is one of them. Why in the world would you ask “how many” when you already know the answer?
Just for the record:
Whenever you ask “How many … ?” , you must have a plural noun after it:
“How many children do you have?”
This has no connection whatsoever with what number might come in the response.
Further, if the number of children is ZERO, the plural form (children) would also be typical in the answer: “I don’t have any children.”