Question

Helium atom has 2 electrons in its valence shell but it's valency is not 2 why

Answer 1

because in its Valence shell it does not contain eight electron's

Answer 2

it is because helium has completed the limit of 2 electrons in the k shell which can take two electrons only.
helium atom has completed it's octet by 2 electrons.
Thus, also Helium atom has 2 electrons but they are used in the particular cell .
also valency of helium =0(it has achieved octet).