The answer is 4. Obviously, 5 would work if you just slice across the prism, and there's no way you can have a prism with a 2-sided base.It must be then less than 5, either 3 or 4. 5 sides is not also possible upon drawing. I can get a rectangular prism to have a 5-sided cross section if you cut near one corner of the base and then exit along the opposite edge.
