In an atom, the total number of electrons having quantum numbers $\mathrm{n}=4,\left[{\mathrm{m}}_{\mathrm{l}}\right]=1 \mathrm{and} {\mathrm{m}}_{\mathrm{s}}=-1/2 \mathrm{is}:$

$\mathrm{n}=4,\mathcal{l}=0,1,2,3$

${\mathrm{m}}_{\mathrm{l}}=1\text{ (only in }\mathrm{p},\mathrm{d}\text{ and }\mathrm{f}\text{-orbitals) }$

$2\text{ electrons in each orbital have }{\mathrm{m}}_{\mathrm{s}}=-\frac{1}{2}$

Hence, total no. of electrons = 6