Arthur Caley was a mathmatician who tried to derive a formula for such a question ,while his work was disproved by modern computors, it is still useful and mostly correct from what I can gather.
first determine how many isomers this "n" alkane has. For any alkane CnH(n+2), where "n" is any integer between 1 and 20, the following sequence represents the number of isomers possible: 1, 1, 1, 2, 3, 5, 9, 18, 35, 75, 159, 355, 802, 1858, 4347, 10359, 24894, 60523, 148284, 366319. In the case of pentane--which has five carbons and therefore represented by the fifth integer of the sequence--the answer is three.