Total number of isomers possible for ${\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{Cl}$  are

1) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2\mathrm{C}l$          2) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{CHCl}-{\mathrm{CH}}_3$

3) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CHCl}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$            4) $\begin{array}{l} {\mathrm{CH}}_3\\ |\\ {\mathrm{CH}}_3-\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_2\mathrm{Cl}\end{array}$

5) $\begin{array}{l} {\mathrm{CH}}_3\\ |\\ {\mathrm{CH}}_3-\mathrm{CH}-\mathrm{CH}-{\mathrm{CH}}_3\\ |\\ \mathrm{Cl}\end{array}$                            6) $\begin{array}{l} {\mathrm{CH}}_3\\ |\\ {\mathrm{CH}}_3-\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_3\\ |\\ \mathrm{Cl}\end{array}$

7) $\begin{array}{l} {\mathrm{CH}}_3\\ |\\ {\mathrm{CH}}_2\mathrm{Cl}-\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_3\end{array}$                       8) $\begin{array}{l} {\mathrm{CH}}_3\\ |\\ {\mathrm{CH}}_3-\mathrm{C}-{\mathrm{CH}}_2\mathrm{Cl}\\ |\\ {\mathrm{CH}}_3\end{array}$

2) $\begin{array}{l} \mathrm{Cl}\\ |\\ {\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{C}}^{*}-{\mathrm{CH}}_3\\ |\\ \mathrm{H}\end{array}$              5) $\begin{array}{l} C{H}_3 Cl\\ | |\\ C{H}_3-CH-C^{*}-C{H}_3\\ |\\ Cl\end{array}$

7) $\begin{array}{l} {\mathrm{CH}}_3\\ | \\ {\mathrm{CH}}_2\mathrm{Cl}{-}^{*}\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_3\\ |\\ \mathrm{H}\end{array}$

(2), (5) and (7) exhibit optical isomerism 

Number of structural isomers - 8

Number of stereo isomers - 6

Total number of isomers - 11

1+2+1+1+2+1+2+1=11