A triangular prism has 5 flat faces.
a rectangle
Pyramid
cube There are a great many solid shapes that have only flat surfaces.
It is a cube or a cuboid
cylinder
a rectangle
Pyramid
You are a square based pyramid
There can be no such shape. If there were a shape with n flat surfaces, then if you sliced off one vertex, you would have a shape with n+1 flat surfaces. So it is always possible to add one more flat surface.
A sphere
5 flat surfaces.
A pentagonal pyramid.
cube There are a great many solid shapes that have only flat surfaces.
A cube
a cylinder
There are 5 flat surfaces.
A paper cup typically has three flat surfaces: the bottom, which is circular, and the two sides which are usually straight and flat. These surfaces are necessary for the cup to hold its shape and be able to stand upright.