The rule which was given by bohr and bury is that the maximum number of electrons that can accommodate in a shell is given by the formula 2 n square where n is the number of shell.

n=1 then, 2 multiplied by 1 square= 2 electrons.

similarly

in L shell 8 electrons

in M shell 18 electrons

in N shell 32 electrons

in O shell 50 electrons