The conductor shell in external electric field will produce inductive charge on its outer surface，and

reach the electrostatic balance. The electric field in its cavity is zero everywhere，this is so called electrostatic shielding. However，if the external electric field is extremely strong，or the number of free electrons in the conductor is

too small，so that the inductive field cannot offset the external field completely. This is so called the upper limit of

electrostatic shielding. In this paper，we simplify this problem as a toy model in which the conductor shell，is illustrated as a pair of parallel metal plates. Through calculation quantitatively，we have found that due to the large amount of free electrons in the metal，any macroscopic conductor devices are far from encountering this upper limit

in non-extreme question.