What your students have to understand is the difference between the subject and the object.
How many people are there in your family?
"people" is the subject here. It is self-sufficient. "What is there?" is the question.
How many glasses of water do you drink a day?
"glasses of water" is the object here. We do something with it. "What do we drink?"
How many glasses of water are consumed by you daily?
Here again we don't need "do", since "glasses of water" is the subject.
One way of solidifying these concepts in the mind of the student is by teaching them the major speech patterns occurring in questioning. At least, that's the way I do it.