if z is no. of atoms in unit cell that represents closest packing sequence ABC ABC . then what is no of tetrahedral voids in unit cell equal to?

ABC...ABC is the type of packing of layers presents in cubic closed packing(ccp) or face centered closed packed (fcc) structure.
In general :
Octahedral voids= no. of atoms in unit cell.
Tetrahedral voids = 2 × no. of octahedral voids.
Number of atoms per unit cell in case of ccp/fcc is 4.
So Octahedral voids = 4 
tetrahedral voids = 2 x 4 = 8

Or if we expain in term of z, which is given in the question then number of tetrahedral voids will be 2z.