2-chlorobutane + Cl_2 → C_4 H_8 Cl_2 (isomers) Total number of optically active isomers shown by C_4 H_8 Cl_2, obtained in the above reaction is ____.

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