That formula will give you the total number of *stereoisomers*, which includes enantiomers and all possible diastereomers. the number of diastereomers and enantiomers is not necessarily equal. An enantiomer is defined by its relationship to its mirror image, while a diastereomer is not. Suppose you have three chiral centers in a molecule. Pick one stereoisomer and you have one enantiomer of that compound and the rest of the stereoisomers are diastereomers relative to the compound you picked (in this case, 6 of them).

Also note that this formula only holds if there are no *meso* diastereomers. If there is a meso form, then you subtract one.