@ kishan
in hcp there are total 6 atoms / sphere arrenged .
the total no of tetrahedral void = 2n where n is the no of sphere .
so, total no of tetrahedral void = 12
no of octahedral void = n = 6 voids
so, there are total 18 voids arrenged in hcp lattice .
HOPE IT CLEARS YOUR DOUBT
ALL THE BEST ..