The compound shown is butane-2,3-diol drawn without stereochemistry. Counting configurational isomers (enantiomers and diastereomers together), the number of stereoisomers is

Butane-2,3-diol has two stereocentres with identical substituent sets, so the $2^2=4$ combinations reduce: $(2R,3R)$ and $(2S,3S)$ are a pair of enantiomers, while $(2R,3S)$ and $(2S,3R)$ are the same meso compound. Total stereoisomers: $2+1=3$.