Why The coordination no of metal crystallizing in HCP structure is 12?

Look at this image.This shows a typical HCP lattice with the ABA layers,which is sliced from the repeating A-B-A-B-..... continuum.

Consider an atom(let us mark the atom as P) in the center of the oranger layer.It is surrounded by 6 atoms in its layer and by 3 atoms in layer B beneath it.(the green layer)

Now,since the lattice is a repeating AB-AB-AB,you should visualise a greeb B layer above the orange layer in which P sits.That means another 3 atoms are neighbors of P.

Finally,

6(orange) atoms in P’s own layer,3(green) above and 3 (green) below. = 12

Which by definition is the coordination number.

As an exercise,you should calculate the number of (full) atoms within the unit cell.This will help picture HCP more clearly.