I found these lines in Mises' Human Action: "A bank can never issue more money-substitutes than its clients can keepin their cash holdings. The individual client can never keep a larger portionof his total cash holding in money-substitutes than that corresponding to theproportion which his turnover with other clients of his bank bears to his totalturnover."
What does Mises mean? Do help me!
Since non-clients won't accept the money-substitues, or if they do, will make good on the claim and trade them in for gold immediately, the bank must have that proportion of exchanges that its clients makes with non-clients in reserve. If everyone was a client of the same bank, that bank could in theory have a 0% reserve since no-one would have any need of the actual money (ignoring any industrial uses of the money).