Shall I contact multiple faculty members in one department?
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.