How many valence electrons are there in an atom with the electron configuration [noble gas]ns2 (n - 1) d10np3?

There are 5 valence electrons in Valence Shell.

Example,

Lets take example of Arsenic with Atomic Number 33.

Its electronic Configuration is,

[Ar] 4s², 3d¹⁰ 4p³

As the valence shell number is 4, so all the electrons present in shell 4 are said to be valence shell electrons and they are 5 in number.