Spinel structure is a CCP with 8 tetrahedral and 4 octahedral voids per unit cell.
Number of O2− ions (CCP) = 4
Nmber of A2+ ions (18 tetrahedral voids) = 1
Number of B3+ ions (12 octahedral voids) = 2
Hence, formula unit = AB2O4
As we can see, the overall charge on the unit cell is zero originally. Now when we replace O2− by X−8/3, the charges change. The net charge on the cation is 8+. The net charge on the anion becomes 323−. Since 8+ can balance 8- of the cation, the rest can be attributed to atomic vacancy.
Unbalanced charge is -8/3 which is nothing but 1 atomic vacancy.