The compound shown is one stereoisomer (X) of 3-chlorobutan-2-ol. Counting all configurational stereoisomers of this constitution, how many of them are diastereomers of X? Give the answer as an integer.

3-Chlorobutan-2-ol has two stereocentres carrying different sets of groups, so no meso form exists and the number of stereoisomers is $2^2=4$: (2R,3R), (2S,3S), (2R,3S) and (2S,3R). X is (2S,3R). One of the other three, (2R,3S), is its enantiomer. The remaining two, (2R,3R) and (2S,3S), differ from X at exactly one centre and are its diastereomers. Answer: 2.