The total supply of Bitcoin is actually not 21 million coins. Because of how the code in Bitcoin actually works, simply cutting the supply in half every halving, there will only ever be 20999999.97690 BTC in circulation...
The supply, due to a quirk in how the programming language (C++) that Bitcoin is written in, was not capped at all originally. In 2214 the entire supply would have started issuing again, resetting the block reward back to 50 BTC per block...
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