The compound shown is 2,3-dimethylbutanedioic acid (2,3-dimethylsuccinic acid) drawn without stereochemistry. The number of stereoisomers is

C2 and C3 each carry $\mathrm{H}$, $\mathrm{CH}{}_3$, $\mathrm{COOH}$ and the other half of the chain, so both are stereocentres with identical substituent sets. $(2R,3R)$ and $(2S,3S)$ form an enantiomeric pair, and $(2R,3S)$ is a single meso compound because of its internal mirror plane. Total: $2+1=3$ stereoisomers.