...to schedule a preliminary oral examination?
ALL.
In my case, that is four folks (3 inside the department and 1 from my minor area).
For a while it was looking like scheduling a date when four busy faculty members (plus one busy graduate student) could meet would be a truly impossible task. But with much flexibility and good humor we were able to pull it off. (THANK YOU!) The big date is November 12.
A while back a new graduate student asked my advice about putting together a five-person committee, with two members acting as co-advisors. I don't remember what I told her, but if anyone asked me now I'd say "NO-NO-NO." The odds of assembling five busy professors for an oral prelim (and then again for a proposal meeting and then again for a final dissertation defense) are probably in the winning-the-lottery range.
...to pass a preliminary oral exam?
In the case of a four-person committee, either all four or three out of the four. According to the University of Minnesota's graduate handbook, even if one committee member votes "pass with reservations," then the vote goes down as "pass with reservations." In this less than ideal situation, a student has to be told what she needs to do to get rid of the "with reservations" part. Even less ideal, a student fails the exam if she does not get the required pass votes. She can be allowed to re-take it, however, if her committee members vote unanimously for this opportunity. (Another rationale for a committee with the fewest allowable committee members.)