The additional ones wouldn't be in a blockchain. The bank would have a database somewhere saying "We owe btbuildem 10 BTC". When you make a withdrawal they wo

The additional ones wouldn't be in a blockchain. The bank would have a database somewhere saying "We owe btbuildem 10 BTC". When you make a withdrawal they would take some out of their massive stock and hand them to you. So long as they have enough to cover withdrawals at any given moment they don't need enough to cover if everyone made a withdrawal at once, any more than they do with dollars/pounds/etc.