I searched for sources for a while now and I agree that what shahzadafzal said. It ranges from sometimes 1.8m to as many as 6 million. Does anyone know a source of which you think the applied methodology to determine lost coins permanently makes sense and is verifiable to a certain degree?
Since these sources vary so widely, I think nobody really has a clue as to how many BTC are lost. When a guy says he threw a hard drive in the trash, even that can't be verified. It is probably true, but it doesn't have to be. Are satoshis coins considered lost permanently in most of these reports?
You can use inactive UTXOs, and different age cut-offs to estimate possible lost bitcoins. With different age cut-offs, you will have different estimations and they are all not accurate.
https://www.bitcoinmagazinepro.com/charts/hodl-waves/There are always very old, and inactive UTXOs suddenly become active again and make your estimations inaccurate. It's only important by knowing a fact that there are lost bitcoins that contribute to higher value of Bitcoin and it's gift for all of us.
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.