How many stereoisomers are possible for 1,2-dimethyl-4-(propan-2-ylidene)cyclopentane?
In this structure the left hand methyl groups can be on the same side of the ring. This is one stereoisomer. This molecule is not chiral and there is only one isomer with the methyl groups on the same side of the ring.

Initially I assumed that there was only one other isomer: the two methyl groups on the opposite side of the ring. But building models sometimes wrecks intuition. The picture below shows the two possible version with trans (relative to the plane of the ring) methyl groups:



So my initial view was wrong: the molecule is chiral and that gives three total stereoisomers.