How many stereoisomers does 1,2,3-trimethylcyclohexane have? Count enantiomers separately; the ring may be treated as planar for counting.

There are three stereocentres, so up to $2^3=8$ configurations, but C1 and C3 are equivalent by the symmetry of the molecule. Using up/down labels for the methyls on C1, C2, C3: $\text{uuu}$ is meso (achiral); $\text{udu}$ is also achiral (mirror plane through C2); $\text{uud}$ (same as $\text{duu}$) and $\text{ddu}$ (same as $\text{udd}$) are mirror images of each other and chiral. That is $1+1+2=4$ stereoisomers.