What is the number of valence electrons in a helium atom?

Helium has two valence electrons.

Valency, the combining capacity of an element, is equal to the number of electrons needed, or required to be removed from the outermost shell of the atom of that element, to have a complete 'octet'. The helium atom has only the 1s orbital in its only shell, and it is completely filled with 2 electrons. Its electronic configuration is 1s2. Therefore, helium and the other inert gases with complete octets normally do not take part in chemical reactions. They are considered as zero-valent elements.