$\text{2-chlorobutane} + \text{Cl}_2 \rightarrow \text{C}_4\text{H}_8\text{Cl}_2 \, (\text{isomers})$
$\text{Total number of optically active isomers shown by} \, \text{C}_4\text{H}_8\text{Cl}_2, \, \text{obtained in the above reaction is} \, \_\_\_\_\_\_.$