In a normal spinel MgAl2O4 the oxide ions form a cubic close-packed array, with Mg^2+ ions in tetrahedral voids and Al^3+ ions in octahedral voids.…

A ccp unit cell contains $4$ oxide ions, hence $4$ octahedral voids (one at the body centre plus $12\times\tfrac14$ on edges) and $8$ tetrahedral voids (two per oxide ion). The formula $\mathrm{MgAl_2O_4}$ has four oxide ions per formula unit, so this cell holds exactly one formula unit: $1\ \mathrm{Mg^{2+}}$ and $2\ \mathrm{Al^{3+}}$. The $\mathrm{Mg^{2+}}$ ion occupies one of the $8$ tetrahedral voids (one eighth), leaving $8-1=7$ empty. The two $\mathrm{Al^{3+}}$ ions occupy two of the $4$ octahedral voids (one half), leaving $4-2=2$ empty. Total unoccupied voids $=7+2=9$.