Question

how many isomers does a pentane have?

Answer 1

\[\text{Three: pentane, 2-methylbutane (isopentane), and 2,2-dimethylpropane}\]\[\text{(neopentane).}\]

Google works wonders sometimes ;)

Answer 2

Formula for pentane is: C5H12

All you're doing is rearranging it's structure around. As long as you still have 5 carbons and 12 hydrogens, it is an isomer. By definition, an isomer is: any of two or more chemical compounds that having the SAME molecular formula but different structural formulas.