Answer
See work below.
Work Step by Step
The values of $m_{\mathcal{l}}$ range from $-\mathcal{l}$ to $\mathcal{l}$, or $2\mathcal{l}+1$ values.
For each of those values, there are 2 possible values of $m_{s}$, which are $-\frac{1}{2},+\frac{1}{2}$.
We see that the total number of states for a particular $\mathcal{l}$ is $2(2\mathcal{l}+1)$.
The āgā subshell has $\mathcal{l}=4$, so there are 2(2(4)+1) = 18 electrons in the subshell.