I know it is a tricky question. What do you think, how many Bitcoins are lost forever and are 'out of the network' by lost private keys?
We can never give an accurate or solid answer to that because we don't have the exact data to know how many were lost.
But I personally think that many Bitcoins has been lost especially in the beginning of the Bitcoin.
Yes, it is not possible to say exactly how many Bitcoin we lost from the beginning. I think I lost too many coins. In the beginning, I lost more bitcoins. But now I can control my play and my bankroll so I hope I am not lost many coins in recent days. The professional gamblers will maintain this balance sheet. The normal gamblers will not care for this.