how many isomers exist in hexane
In this case they are n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. Any formula that goes by CnH2n+2 is an alkane. There isn't a formula you can use to find out how many isomers there are for any given class of molecules with a specified number of carbons. You just have to think very systematically when you try to find them all.