The radius of ${\mathrm{Na}}^{+}$ is $95 \mathrm{pm}$ and that of ${\mathrm{Cl}}^{-}$ ion is $181 \mathrm{pm}$. Hence the co-ordination number of ${\mathrm{Na}}^{+}$will be

The ratio of the radius of cation and anion helps determine the coordination number.

It is provided in the question that radius of cation is $r^{+}=95 \mathrm{pm}$ and the radius of anion is ${\mathrm{r}}^{-}=181 \mathrm{pm}$

Taking ratio we have:

$\frac{{\mathrm{r}}^{+}}{{\mathrm{r}}^{-}}=\frac{95}{181} \mathrm{pm}$

$\frac{{\mathrm{r}}^{+}}{{\mathrm{r}}^{-}}\approx 0.524 \mathrm{pm}$
Because the ratio of the radius is in between $0.414$ to $0.732$, the coordination number of cation is 6.