Cubic close packed structure contains one atom at each corner of an unit cell and one at face centre of each unit cell. Each unit cell consists of 8 small cubes.
Each small cube contains 4 atoms at its alternate corner when these atoms are joined to each other lead to creation of a tetrahedral void as shown in the figure below.




(a) Eight tetrahedral voids per unit cell of cep structure



(b) One tetrahedral void showing the geometry

Since, there are total 8 smaller cubes present at one unit cell and each smaller cube has one tetrahedral void hence, total number of tetrahedral void present in each unit cell is equal to eight.