Number of isomeric products formed by monochlorination of $2$-methylbutane in presence of sunlight is _______.

Monochlorination means the addition of a chlorine atom. When chlorine atom is added to the below structure. Chlorine will substitute at one of the hydrogens

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.

$\therefore$ Number of isomeric products $=6$