In HCP arrangement of atoms, coordination number of atoms in the middle layer is

 

In hcp (hexagonal closed packed) arrangement of atoms, the unit cell is made up of three layers of atoms. Top and bottom layers contain six atoms at the corner of a hexagon and one atom at centre of the hexagon. The middle layer contains three atoms. The middle layer nestled between the atoms of top and bottom layers, hence it is called closed packed structure.

In hcp arrangement, the coordination number of atoms is 12 and hence the coordination number of the atoms in middle layer is also 12.