There are currently approximately 716,000 bitcoins (4.1% of circulation), 7 million ether (6.8% of circulation) and 1.2 billion tether (63.5% of circulation) held in the publicly known wallets of crypto exchanges according to research conducted by The Block (see the table). It’s important to note that:
Coinbase said that it migrated as much as 5% of bitcoin in circulation and 8% of all ethereum in December due to the scheduled wallet maintenance. But Coinbase’s wallets are not publicly known so their claims couldn’t be verified. BitMEX’s wallets reportedly hold at least 1% of bitcoin in circulation. Following these two claims, at least 10% of all bitcoin in circulation is currently held in the wallets of exchanges.