According to the question, ${\mathrm{Z}}_{\mathrm{eff}}$​ For HCP electrons = $6$

Number of unit cells in HCP = $\frac{1}{6} x 6.023 x 10{ }^{23}=1.00 x {10}^{23}$​

In one unit cell $12$ Tetrahedral and $6$ Octahedral voids are present.

Total voids =$18$ (in one unit cell)

Total number of voids = $18 x 1.00 x {10}^{23}=18 x {10}^{23}$