Source of the no. of isomers in alkanes, and other organic molecules
Not counting stereoisomers and enantiomers, the number of isomers of the first ten alkanes is as follows:


Methane: 1 
Ethane: 1 
Propane: 1 
Butane: 2 
Pentane: 3 
Hexane: 5
Heptane: 9 
Octane: 18 
Nonane: 35 
Decane: 75


Source: http://elib.mi.sanu.ac.rs/files/journals/tm/21/tm1121.pdf and Wikipedia articles for these alkanes. The PDF has the number of isomers for the first 30 n-alkanes.

The number of isomers possible with an n-carbon alkane can be calculated by a recursive combinatorial function; however, as the linked paper states, this math isn't for the weak. The number of structural isomers of a 30-carbon alkane is over 4 billion, and a 50-carbon alkane has over 1031 possible isomers. Even organic chemists and petrochemists usually don't bother to identify any one isomer beyond about the pentane mark. Some of the more symmetrical isomers are interesting on their own, since a few of the more compact ones (like the n-methyl types) have markedly different behaviors such as boiling point from longer isomers (for example, tetramethylbutane, an isomer of octane notable for its relatively narrow liquid window). The 2,2-dimethylpropane isomer of pentane is visually interesting because of its isohedral geometry, and likewise the tetraethylmethane/diethylpentane isomer might find interest among a few supremacist groups but is otherwise just another 9-carbon alkane.