Faculties often discuss among themselves. There is no harm in contacting more than one professor from the same department, but be very careful not to send a generic mail to all of them or saying something like "You are my best choice" to every professor. Try to be specific to professors research work. It's not uncommon that people have more than one research interest and professors do understand that. Clarify your interest and explain how your interest matches with professor's research work, that will lead you to write unique mail to each professor.