How much US cash is held out of the country? A lot. Christopher Neely at the St. Louis federal Reserve explains:

The U.S. dollar has been the most widely used international currency since 1945.¹ The dollar is the most traded currency on international financial markets, dollars comprise 60% of official reserves (i.e., foreign exchange reserves), and many traded goods, such as oil, are commonly invoiced in dollars. In addition to these commercial uses of the dollar and dollar-denominated assets, individuals in many parts of the world hold U.S. currency, i.e., paper money, both as a store of value and as a medium of exchange.² This blog post³ explains the widespread use of U.S. currency and provides some simple, back-of-the-envelope calculations on the size of some of the benefits to Americans.

The rest of the world holds a great deal of U.S. currency, i.e., cash. Although the amount can’t be precisely tracked, the Federal Reserve Board of Governors recently estimated that foreigners held $950 billion in U.S. banknotes at the end of the first quarter of 2021, or about 45% of all Federal Reserve notes outstanding, including two-thirds of all $100 bills. Overall holdings of U.S. currency have grown rapidly, however, and overseas holdings of Federal Reserve notes would now be worth closer to $1.1 trillion if such holdings are still half of all U.S. currency.