20 is a the sum of the first 4 triangle numbers (1,3,6,10)
There were 20 shillings in the pound under the old money system.
20 is a score, a unit often used in the Bible.
An icosagon is a 20-sided polygon.
An icosahedron is a Platonic solid with 20 faces.
A dodecahedron, a 12-faced polyhedron, has 20 vertices (each face is a pentagon).