Euler says:

If one tries to tessellate the sphere surface with solely hexagon. Let $x$ be the number of hexagon, then:

This leads to contradiction.

To make tessellation possible, we have to add $y$ pentagons, so that:

So we can solve and get $y = 12$.

So a sphere surface can be tesellated with 12 pentagons plus (many) hexagons.

See also: