How many stereoisomers does 1-bromo-3-chlorocyclohexane have? (Count enantiomers and diastereomers as distinct; conformers are not counted.)

There are two stereocentres (C1 and C3) and the molecule has no symmetry because Br and Cl differ, so $2^2=4$ stereoisomers: cis (1R,3S)/(1S,3R) and trans (1R,3R)/(1S,3S), i.e. two enantiomeric pairs. The count of 3 applies only to identical substituents.