Question

How We can say that noble gases have 8 electrons in its outer shell?

Answer 1

Inert is a much better term, in my opinion. “Noble” infuses the elements with something really special, but really it is simple. The helium’s first (and only) shell is full at two, just the other inert gases have 8 in their final electron shell, which is also full. Full outer electron shells don’t want to play with other elements
Argon has 18 electrons overall and 8 has in the outer shell.