and is there any formula that I can use to determine the number or I just have to know it ?
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There are 5. There is no formula to follow, you either have to just know it or you can try drawing out all possible structural isomers and noting all the different ones. You can know for sure that C6H14 is an alkane (has only single bonds) because it has the formula of CnHn+2. So this would make determining the number of isomers by drawing them out a little more possible.