But even then, do certain exchanges have a certain fixed amount that needed to be on the exchange so trading can happen?
Of course. I assume only a small percentage is held on the exchange's hot wallet(funds being actively used), and most is on their cold storage.
And if that be so, then how is it helpful to be on the exchange after being hacked?
Not sure what you mean here? People should only use exchanges to
exchange funds, not
store their funds.