How many stereocentres (sp${}^3$ carbons bearing four different groups) are present in 3-methylcyclohex-1-ene?

Only C3 carries four different groups: H, CH${}_3$, the vinylic C2 (=CH) and the CH${}_2$ at C4. C4, C5 and C6 each have two H atoms, and C1 and C2 are sp${}^2$. Hence the count is 1.