Therefore, maximum potential difference that can be produced in between the plates of the capacitor of plate separation $\mathrm{d}$ is

$\mathrm{V}_{\mathrm{m}} =\mathrm{E}_{\mathrm{m}} \cdot \mathrm{d} $

$=3 \times 10^{6} \times 2 \times 10^{-3} $

$=6 \times 10^{3} \mathrm{\,V}$

Also the capacitance of the capacitor is $\mathrm{C},$ then

the maximum possible charge on the plates of the capacitor

$\mathrm{q}_{\mathrm{m}}=\mathrm{CV}_{\mathrm{m}}=\frac{\varepsilon_{0} \mathrm{A}}{\mathrm{d}} \mathrm{V}_{\mathrm{m}}$

$=\frac{8.86 \times 10^{-12} \times 20 \times 10^{-4}}{2 \times 10^{-3}} \times 6 \times 10^{3}=53.16 \times 10^{-9}$

$\approx 53 \mathrm{\,nC}$