Why can we say that charge of any body is always an integral multiple of $'e'$ ?

If the protons and electrons are the only basic charges in the universe, all the observable charges have to be integral multiples of $e$.

If a body contains $n_{1}$ electrons and $n_{2}$ protons, the total amount of charge on the body is $n_{2} e+$ $n_{1}(-e)=\left(n_{2}-n_{1}\right) e$

where $n_{1}$ and $n_{2}$ is integral multiples.

and their difference $=n_{2} e-n_{1}(-e)$

$=\left(n_{2}+n_{1}\right) e$ also an integer

Thus, the charge on any body is always an integral multiple of $e$ and can be increased or decreased also in steps of $e$.