Stereoisomers are compounds that are identical in composition and connectivity but have a different spatial arrangement of atoms around the central atom. A molecule having multiple stereocenters will produce many possible stereoisomers. In compounds whose stereoisomerism is due to tetrahedral (sp) stereogenic centers, the total number of hypothetically possible stereoisomers will not exceed 2, where n is the number of tetrahedral stereocenters. However, this is an upper bound because molecules with symmetry frequently have fewer stereoisomers. 
The stereoisomers produced by the presence of multiple stereocenters can be defined as enantiomers (non-superposable mirror images) and diastereomers (non-superposable, non-identical, non-mirror image molecules). Enantiomers and diastereomers are produced due to differing stereochemical configurations of molecules containing the same composition and connectivity (bonding); the molecules must have multiple (two or more) stereocenters to be classified as enantiomers or diastereomers. Enantiomers and diastereomers will produce individual stereoisomers that contribute to the total number of possible stereoisomers. 
However, the stereoisomers produced may also give a meso compound, which is an achiral compound that is superposable on its mirror image; the presence of a meso compound will reduce the number of possible stereoisomers. Since a meso compound is superposable on its mirror image, the two "stereoisomers" are actually identical. Resultantly, a meso compound will reduce the number of stereoisomers to below the hypothetical 2 amount due to symmetry.
Additionally, certain configurations may not exist due to steric reasons. Cyclic compounds with chiral centers may not exhibit chirality due to the presence of a two-fold rotation axis. Planar chirality may also provide for chirality without having an actual chiral center present.