The best we can do is guess. That's a sad reality about economics. Or happy... I can see reasons why that's a happy reality too. Most guessing for the USD (no idea about btc) is based on loan origination. There's a "velocity of money," which is also a best guess, which you multiply by the loans created by banks minus loans closed out, to get the guesstimate of how much money is circulating. Vaguely... I haven't done an exercise for this in over ten years. Bitcoin is going to be even harder to guesstimate. Its probably right to assume most transactions aren't reported, and knowing which tx coming out of one utxo is the spend and which is the change is probably mostly guessing.