The maximum number of electrons that can be accommodated in all the orbitals for which $l=3$ is

The number of orbitals for  $\mathrm{l}=3$ is $(21+1)=(2\times 3+1)=7$

Now, each orbit can accommodate 2 electrons.

So, number of electrons $=2(21+1)=2(2\times 3+1)=2\times 7=14$

Therefore, number of orbitals in the given shell $=7$. 

the maximum number of electrons that cannot be accommodated in these orbitals`$=2\times 7=14$` [`because` each orbital can accommodate a maximum of 2 electron].