Question

how do banks maintain cash reserve?

Answer 1

Banks Always maintain a amount of Cash of deposits with themselves for the withdrawals
Banks know that all the depositers will not withdraw at the same day and hence some of the money is circulated while the rest is kept for deposits
and thus this way they maintain cash reserves

Answer 2

Banks keep 15 percent of the deposits with them so that they can pay money to depositors who come on any given day to withdraw money