The close packing sequence ABCABCABC... represent the cubic close packing structure. Here, Z is the number of atoms in the unit cell.
In ccp/fcc, there are two tetrahedral voids on the every body diagonal of the cube/unit cell.
Fcc has total 4 body diagonals. Thus, totally 8 tetrahedral voids present inside the unit cell.
Also, number of atoms in fcc/ccp arrangement is 4
Hence,
\(\text{No. of atoms per unit cell }= Z\)
\(\text{No. of tetrahedral voids }= 2Z\)