There is no sensible answer to this question. You can put any
amount of charge on a blob of aluminum sitting in a vacuum, or
surrounded by an ideal insulator. Why not? If you put an awful
lot of electrons on a blob of aluminum sitting in a vacuum, the
electrons will eventually start shooting off by thermionic
emission, and most of the excess charge will be gone after,
let's say, 1 day. If you put even more electrons, most of them
will be gone after 1 millisecond. But there is no "maximum"
really, just a gradual speed-up of the discharging. Even 1
excess electron will not be stable for eternity. If you subtract
electrons instead of adding them, certainly nothing will happen.
Well, I guess positively-charged atomic nuclei could fly off if
the charge was significant enough. Again, this process does not
let you say that a certain amount of charge is "the maximum
possible", it's just a process that happens more and more
frequently as the charge increases. If you add or subtract an
awful lot of electrons from a blo