20

Number
$20$ (twenty) is:


 * $2^2 \times 5$


 * The $4$th tetrahedral number, after $1, 4, 10$:
 * $20 = 1 + 3 + 6 + 10$


 * The $4$th semiperfect number after $6, 12, 18$:
 * $20 = 1 + 4 + 5 + 10$


 * The number of faces on an icosahedron


 * The number of vertices on a regular dodecahedron

Also see

 * Definition:Vigesimal System