Question

how to find the no. of isomers for the given molecules...?

Answer 1

There are no molecules, thus no isomers.

Answer 2

Where's the molecular formua?

Answer 3

I am attaching a file that should be helpful.

Answer 4

well for any molecule you have to consider these:
Structural Isomers
Sterioisomers(includes enantiomers and diasteriomers)
and
Meso Compounds

In that file they are only considering structural isomers as isomers which I think is wrong. I mean for example decane has structural isomers that can be flipped to get an enantiomer or have part of it flipped to get a diastereomer.