Question

Why n-butane is two isomers of monochloro derivatives are possible?

Answer 1

Yes, there are only two isomers for mono-chloro derivatives of n-butane, because even though you think you got more than 2, you must have gone wrong somewhere, because you will get the mirror image of the isomers, which are the same.

Thus the only 2 mono-chloro derivatives of n-butane are
1) 1-chloro butane
2) 2-chloro butane

Hope it helps!