Many Bitcoins are lost, it's hard to calculate how many.
How is the formula for calculating the lost Bitcoin?
You can estimate lost bitcoins by very long time inactive UTXOs, but it's estimation and it's not accurate number for actual lost bitcoin. Inactive Bitcoin UTXOs many years ago can turn to be active and these coins are not lost bitcoins.
You can use HOLD waves and make your estimation.
https://www.bitcoinmagazinepro.com/charts/hodl-waves/Lost Bitcoin: 3.7 million Bitcoin are probably gone forever. You can uncertainty in the article title, it's "probably gone forever".