The amino acid isoleucine, 2-amino-3-methylpentanoic acid, is shown drawn with wedges. How many of its stereocentres have the $(S)$ configuration? Give an integer.

Isoleucine has two stereocentres. At C2 the priorities are $\mathrm{NH}{}_2>\mathrm{COOH}>\mathrm{C}{}_3>\mathrm{H}$ and the drawn arrangement is anticlockwise with $\mathrm{H}$ away, so C2 is $(S)$. At C3 the priorities are $\mathrm{C}{}_2$ (attached to N,C,H) $>\mathrm{C}{}_2\mathrm{H}{}_5$ (C,H,H) $>\mathrm{CH}{}_3>\mathrm{H}$ and the drawn arrangement is also $(S)$. The compound shown is $(2S,3S)$-isoleucine, the natural L-isoleucine, so both stereocentres, i.e. 2, are $(S)$.