According to the NCERT text, for a given value of the azimuthal quantum number ($l$), the magnetic orbital quantum number ($m_l$) can have $(2l + 1)$ values (ranging from $-l$ to $+l$).

Let $n_m$ be the number of permissible values of $m_l$. $n_m = 2l + 1$

To find the relation for $l$, we rearrange the equation: $n_m - 1 = 2l$ $l = \frac{n_m - 1}{2}$