There are only 21 million total bitcoins that can be mined, what will happen if all bitcoin is already mined?
once all 21 million have been mained, there will never be any new bitcoins (unless a change to the protocol is made to increase the supply).
Most coins have already been mined though. There are already like 16 million in existence. Now we need to make Bitcoin more spendable, because at the moment it still can't be used to buy most things.