Well that's a good question to investigate! I guess that is very difficult to know at this point because no one gave report if lossed bitcoin. I think that we will only be able to find how many BTC are lost after they are all mined. Or we can probably research the blockchain legldger and find a pattern that could lead us to see how many BTC could be outside of the. And many user forget wallet id so some amount is loosed in that way