(b) is correct. It takes an act of Congress. The Constitution itself does not give any specific number.
Congress has changed the number many times. It has been five, six, seven, nine, and ten over the years. A case can be made for having an even number, say 10, to prevent the many 5-4 decisions we have had. A decision would then require a two-person majority. In the case of a tie (which can happen now if one justice recuses himself or herself), the lower court ruling stands.