If a metal has a BCC crystal structure, the coordination number is 81 because :

each atom touches four atoms in the layer above it, four in the layer below it and one in its own layer

each atom touches four atoms in the layer above it, four in the layer below it and none in its own layer

two atoms touch four atoms in the layer above them, four in the layer below them, and none in their own layer

each atom touches eight atoms in the layer above it, eight in the layer below it, and none in their own layer