Total number of isomeric compounds (including stereoisomers) formed by monochlorination of $2$-methylbutane is ______.

Monochlorination means the substitution of a chlorine. When chlorine atom is added to the below structure, chlorine will substitute at one of the hydrogen.

Mono chlorination of $2$-methyl butane gives four structural isomers.

Here only two, i.e., I and III are optically active. Since each optically active compound has two enantiomers, therefore, total six isomeric compounds are possible.