The maximum number of electrons that can be accommodated in the M shell are

The maximum number of electrons that can be accommodated in an M shell are 18.
There will be a limit of electrons a shell can hold. M shells can hold a maximum of 18 electrons only. It is calculated using the formula 2n². The value of n for M-shell is 3.

Hence, the number of electrons in the M-shell will be $2\times {\left(3\right)}^2$ = $2\times 9=18$