What are N and M ?

In these, (iii) and (iv) are enantiomers and (v) and (vi) are also enantiomers to each other. Thus total six isomers will be possible for $N$. In these only four isomers will be obtained on fractional distillation without resolution because $d$ and $l$ isomers (i.e., (iii) \& (iv) and (v) \& (vi)) cannot be separated by fractional distillation. Hence $N=6$ and $M=4$.