Shall I contact multiple faculty members in one department? In the process of a PhD application, if contacting the faculty members are encouraged, shall I contact more than one faculty members saying that I am interested in his/her research?

Not all professor list their on-going project so a match in interests cannot usually be determinated before talking.

If I am contacting multiple members shall I disclose that I am contacting a lot of them? Is it impolite?
If you are interested in the research topics of several members of a department, you can contact all of them. Everyone knows that people apply to several topics and several universities. You should always make sure that each message is "personalized" and explains why you are interested in that particular topic. Generic emails are more likely to be ignored.

If the recipients are likely to talk to each other (as is the case when they are in the same department), I would mention in the emails that you have contacted some other members of the department. They will probably find it out anyway, and it looks more honest if you say it in advance.