There is a formula for finding the number of faces, edges, and vertices of any convex polyhedron. So if you don't know the shape you can use the formula.
V + F - E = 2
12 + F - 18 = 2
F - 6 = 2
add 6 to both sides
F = 8
CHECK:12 + 8 - 18 ??=?? 2
20 - 18 = 2 checks
