It is a basic result in electrostatics that a charge $q$ in an arbitrary cavity of an ideal conductor will generate a total charge $-q$ on the surface of the cavity in such a way that the electric field is cancelled outside the the cavity (but only within the conductor). However, if the charge $q$ is the charge of an electron, and therefore the fundamental quantum of charge, how can opposing charge arrange itself in a symmetric way? Is the field cancelling nature of ideal conductors only valid when the amount of charge in play is large enough that quantization of charge is negligible?
You are asking if the charge is an electron, how can an opposing charge arrange itself in a symmetric way?
In a simple atom, the electron field that exists around the proton as per QM, arranges itself in a symmetric way so, that the atom itself is EM neutral.