Question

what is the crieteria to decide number of electrons in a shell?

Answer 1

no of electron is equal to the atomic no of the element

Answer 2

Each shell can hold a specific number of electrons only. You can find that by 2n^2 where is the shell number.
This means that the electron present in the shell would be
k shell = 2(1)^2 = 2shell
L shell =2(2)^2 = 8shell
M shell = 2(3)^2 = 18shell
N shell = 2(4)^2 = 32shell
and so on...