Calculating the number of enantiomers

Total no. of enantiomers = [No. of Stereoisomers] - [No. of diastereomers]

In general, number of diastereomers of a molecule is calculated as 2n, where n is number of chiral centers in the molecule (chiral carbon:asymmetric carbon that exhibits optical isomerism).
The relation holds true when the molecule does not have any meso forms (diastereomeric forms).
I. A molecule with one asymmetric carbon (stereocenter) forms 2 mirror images.
II. A molecule with two asymmetric carbons gives 4 configurations that may or may not be mirror images of each other.

Examples:

Tartaric acid:
The acid has 2 asymmetric carbons that gives four possible configurations.
Among these, two isomers are meso type (optically inactive), while the remaining are D - and L - isomers that are mirror images (enantiomers).
The possibilities continue to multiply with more asymmetric centers in a molecule.
A meso compound is an achiral compound that has chiral centers.
Although achiral compound has two or more stereocenters, it is optically inactive and gives non-super imposable mirror images.
Limonene
*An enantiomer has orange smell while the other has lemon smell.

Carvone

Carvone
*(R) - Carvone is the principal component of caraway seed oil and is responsible for its odor.
*(S) - Carvone is the principal component of spearmint oil and is responsible for its odor.

Ibuprofen

Ibuprofen
*In Ibuprofen, only the(S) isomer is effective as an anti-inflammatory agent.
Therefore an enantiometrically pure drug would work more effectively than a racemic mixture.

Thalidomide

Thalidomide
One of the enantiomers (the R form) alleviates morning sickness, the other is a potent mutagen that causes birth defects.

Pencillamine

Pencillamine
*(S) - Pencillamine is a potent drug for chronic arthritis, whereas (R) - Pencillamine is not only useless for therapeutic action, but is also highly toxic.

Methyldopa

Methyldopa *Only the (S) isomer of methyldopa acts as antihypertensive drug; the other isomer has no effect.
The complex nature of organic compounds - their tendency to form very large molecules and their ability to occur as a wide variety of different types of isomers - leads to the enormous variety of organic chemicals currently known to exist.