Total number of isomers (including stereoisomers) obtain on monochlorination of methylcyclohexane is

Compound A and B given above have no chiral centre and doesn't show geometrical isomerism.

Compound C and D given above have two chiral centres each and each of them forms 4 optical isomers.

And compound E has 2 E and Z isomers due to restricted rotation of groups.

Hence, the total number of  isomers will be 12.