6.7% of the entire circulating supply of Bitcoin is held in wallets belonging to eight major cryptocurrency exchanges
Source: https://cointelegraph.com/news/nearly-10-billion-in-btc-is-held-in-wallets-of-8-crypto-exchanges
Nearly $10 Billion in BTC Is Held in Wallets of 8 Crypto Exchanges
October 8, 2019
CoinTelegraph