The number of spheres (atoms, molecules or ions) directly surrounding a single sphere in a crystal is called the coordination number. In Cubic Close Packing (ccp or fcc) the coordination number if 12. In this case the two dimensional layers are arranged in ABCABC....... pattern as shown below.
It can be seen from the diagram that the atoms in layer C are arranged below second type of trigonal voids of layer B. In ccp a particle is touched by six particles of the same layer and 3 particles each from the layers above and below it. Thus a total of 12 particles touch every particle ∴ co-ordination no. is 12.