If an atom, the total number of electrons having quantum number n = 4, $\left|{\mathrm{m}}_{\mathcal{l}}\right|=1$  and ${\mathrm{m}}_{\mathrm{s}}=-\frac{1}{2}$ is

$\mathrm{n}=4,\mathcal{l}=0, 1,2,3; {\mathrm{m}}_{\mathcal{l}}=+1,-1; {\mathrm{m}}_{\mathrm{s}}=-\frac{1}{2}$

For

$\begin{array}{l}\mathcal{l}=0,{\mathrm{m}}_{\mathcal{l}}=0\\ \mathcal{l}=1,{\mathrm{m}}_{\mathcal{l}}=-1,0,+1\\ \mathcal{l}=2,{\mathrm{m}}_{\mathcal{l}}=-2,-1,0,+1,+2\\ \mathcal{l}=3,{\mathrm{m}}_{\mathcal{l}}=-3,-2,-1,0,+1,+2,+3\end{array}$

There will be six electrons with ${\mathrm{m}}_{\mathrm{s}}=-\frac{1}{2}$