From the below image it is clear that two isomers are produced if the reactant complexion is a cis-isomer, as well as only one isomer, is produced if the reactant complex is a trans-isomer.
Two isomers are produced if the reactant complex ion is a cis-isomer because the cis isomer can further form facial and meridional isomers.
Only one isomer is produced if the reactant complex ion is a trans-isomer because it cannot form any isomeric isomers.

Trans → only meridonial
Cis → fac and mer