I just looked it up and the current reserve requirement is 0%. This means banks are required to hold no cash to back any deposits. I wonder what would happen if there was such a run that a bank had no one to sell their liabilities to? Would your mortgage and loans just go away?
The rules were changed in 2020 to make the reserve requirements 0.
The Fed would probably do another round of Quantitative Easing and purchase the assets, giving banks money in return.
We're living in very dangerous economic times.