The compound shown is 4-ethyl-3-methylheptan-2-ol drawn without stereochemistry. How many stereocentres (sp3 carbons bearing four different groups) does it have? Give an integer.

A stereocentre needs four different groups. C2 has $\mathrm{H}$, $\mathrm{OH}$, $\mathrm{CH}{}_3$ and the rest of the chain. C3 has $\mathrm{H}$, $\mathrm{CH}{}_3$, the $\mathrm{CH}(\mathrm{OH})\mathrm{CH}{}_3$ side and the C4 side. C4 has $\mathrm{H}$, ethyl, propyl and the C3 side; ethyl and propyl are different. So there are 3 stereocentres, and no symmetry, so $2^3=8$ stereoisomers.