How many camera can be taken?

Hi, I have a question. My parents in law will come here to visit us and we plan to buy digital camera to my sisters and brothers in law, also buy a camera recorder for my parents in law. However, sisnce my hu*****and has two sisters and one brother, we need to buy four. I wonder if my parents in law can take all these four to china without duty? or they can only take one? Thanks for any information.