How many electrons in an atom may have the following quantum numbers?

$A$: $n=4,m_s=-\frac{1}{2}$

$B$: $n=3,l=0$

$A$:

We know that,
Maximum number of electrons in an atom for a value of $n=2n^2$
$\therefore \text{for}n=4$, maximum number of electrons possible $=2(4{)}^2=32$
Now each fully filled orbital has equal number of electrons with $m_s=-\frac{1}{2}\text{and}{m}_s=+\frac{1}{2}$
Therefore, maximum number of electrons having$n=4\text{and}{m}_s=-\frac{1}{2}=16$

$B$:

$n=3,l=0$ indicates $3s$ orbital. s orbital can have a maximum of $2$ electrons.

$\therefore \text{maximum number of electrons having}n=3\text{and}l=0\text{is}2$