Is it ok to contact multiple potential supervisors?
Yes, you can contact several. But before you contact anyone, make sure that your interests and background align with their specialties. If your letter states that you are exploring your alternatives and seeking an advisor, rather than specifically asking if they will be your advisor, it will be easier for you to "disappoint" one or more of them by choosing another, assuming you get a lot of interest. "I'm interested in potentially being your student and ..." 

If it is possible for you to actually visit the institution, you could try to have face to face visits with some of the faculty. Try to visit them in roughly the same order of your interest in them and if you "hit it off" with one, you are done.