Five. They are:
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A cube, a dodecahedron, an icosahedron amongst regular polyhedra. Many irregular polyhedra, including a prism.
A polyhedron is a solid with flat faces - a cube is just one of many different examples of regular polyhedra - otherwise known as platonic solids.
They are all convex, they are all polyhedra and they are all regular.
There are different formulae for different polyhedra and these depend on what information about the polyhedron is given.
A regular polyhedron (other than a hexahedron), any pyramid, as well as most irregular polyhedra.