Cyclohexane is an alkanewith its formula C6H12. But an alkane has the formula CnH2n+2. On this basis, cyclohexane should have the formual C6H14. Why is this?

Cycloalkane is a different category of saturated organic compound which consist of atleast one cyclic ring. These compound are quite different in terms of their structure, as they are not open chain alkanes therefore their general formula is C_nH_2n bit different from alkanes. 
For example cyclo pentane, cyclohexane, cyclopropane etc. 
For more information, kindly refer to the following link 
https://www.meritnation.com/ask-answer/question/cyclohexane-is-an-alkane-right-then-why-doesnt-it-follow/carbon-and-its-compounds/1614212