give me structure of 2,3-dimethylbutane. and explain me that how u derive the formula of this isomers of hexane C₆H₁₄

The structure of 2,3- dimethyl butane is 

The 5 isomers of hexane are:

H₃C-CH₂-CH₂-CH₂-CH₂-CH₃

_n-hexane

_{2-methylpentane}

_{3-methylpentane}

_{2,2- dimethylbutane}

_{2,3-dimethylbutane}

There is no formula to calculate the number of isomers a compound has.