Let's talk about circulating supply and total supply.
Different data websites may use different criteria for calculating “circulating supply,” so the figures for the same coin may vary from site to site.
One thing that's easy to overlook: when a coin's circulating supply is only a small fraction of its maximum supply, the pressure from future releases could be significant. In that case, it's worth looking at the unlock schedule as well.
When comparing the valuations of two coins, first check whether you're comparing their circulating market caps or their FDVs. Different metrics can lead to very different results.
$BTC has a fixed maximum supply of 21 million coins, and the number already mined is gradually approaching this limit.

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