The compound shown is one stereoisomer (X) of 3-chlorohex-4-en-2-ol, $\mathrm{CH}{}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{CHCl}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_3$. Counting all configurational stereoisomers (geometrical and optical together) of this constitution, how many are diastereomers of X? Give the answer as an integer.

The constitution has two stereocentres (C2 and C3, carrying different groups) and one stereogenic double bond (C4=C5). The molecule has no symmetry, so the total number of stereoisomers is $2^3=8$. X itself is (2S,3S,4E). Its mirror image, (2R,3R,4E), is its enantiomer (reflection does not change $E$/$Z$). Every other stereoisomer is neither X nor its mirror image, so the number of diastereomers of X is $8-1-1=6$.