There is technically not 21 million coins.

snvrky
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If you remember the Bitcoin overflow incident, you would know that 0.5 bitcoin was burned during a transaction. When they fixed the code and forked the chain, those coins were actually completely destroyed forever. There are also dormant addresses, but theoretically, those coins could still be recovered thousands of years from now. There is actually 20999999.5 bitcoin that will ever exist. That's just a fun fact about Bitcoin.