I dare to assume that by adding together the wallets of all people who think they have bitcoin, we will exceed 21 million even today (with 18 million real bitcoins in circulating supply).
Unfortunately I will completely agree with this considering how many BTC forks we have had so far and the fact that we have a certain percentage of crypto users who bought just that forks thinking they are buying BTC. One day they will be quite surprised, I'm sure.
What I believe when it comes to CMC is the data on total/available supply and price which then gives the total market cap. Speculations about how much BTC has actually been lost are nothing but speculations, and are mostly based on the fact that BTC has not moved from some address x years, which is really pretty stupid.
Until recently, we considered
these BTCs lost, but then someone showed 2 months ago that this is not the case - so I think that in fact a very small amount of BTC is completely lost.