The compound shown is 2,3,4,5-tetrachlorohexane drawn without stereochemistry. Counting only configurational isomers, how many of its stereoisomers are achiral (meso forms)? Give an integer.

The molecule $\mathrm{CH}{}_3\mathrm{CHCl}-\mathrm{CHCl}-\mathrm{CHCl}-\mathrm{CHClCH}{}_3$ has four stereocentres and is constitutionally symmetric about the C3-C4 bond. Of the $2^4=16$ configurations, those in which C2/C5 and C3/C4 have opposite descriptors (R,R,S,S read from one end and S,S,R,R from the other) have a mirror plane, and these give 2 distinct meso compounds: $(2R,3R,4S,5S)$ and $(2R,3S,4R,5S)$. The remaining 8 chiral isomers form 4 enantiomeric pairs, so the total is 10 stereoisomers, 2 of them meso.