Which question is correct ?
What page is it ?
Which page is it ?
Are you sure you know the difference between the two?
I ask, because we can't know for sure unless we have the context.
Most commonly, the appropriate form of the question would be, "What page is it?"
If students were asked to study a section in a text book, pages 41 to 56, and next day a student says he had problems understanding what was meant in a particular paragraph, the teacher might say, "Which page is it on?" and be just as grammatically correct in doing so, from his/her point of view.