By default, the number of mers, Z, in a unit cell is worked out from the empirical formula. Thus, if a unit cell has the empirical formula C24H24, then Z=24. In those cases where the empirical formula should not be used, e.g., benzene, C6H6, the number of mers can be explicitly defined by use of Z=n, here Z=4. Be careful to distinguish between the number of mers in a unit cell and the number in a cluster - Z=n applies to the number in the unit cell.
If Z=n should be used, but is not used, then the heat of formation per formula unit will be in error by a factor, in the case of crystalline benzene, the ΔHf per formula unit would be 1/6 what it should be.