Estimates from on-chain analyses suggest that between 2.3 million and an incredible 7.8 million BTC (roughly between 11—37% of total supply), may have vanished forever, trapped in lost wallets, forgotten keys, or in addresses abandoned due to unexpected deaths.
It is my first time to know that about 7.8 million bitcoins possibly lost forever.
Previous reports and estimations are only about less than 4 million bitcoins lost.
3.7 million Bitcoin are probably gone forever.You can estimate different figures for possible lost bitcoins by using different definitions on inactive years for identifying possible lost bitcoins. You can use this UTXO chart and your definitions for getting your own numbers.
https://www.bitcoinmagazinepro.com/charts/hodl-waves/