Individuals should completely disabuse themselves of the notion that there will ever be 21 million Bitcoins
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How is the 21 million Bitcoin cap defined and enforced by Jameson Lopp.
Taking lost and inaccessible coins into consideration, as well as Satoshi's 1M BTC lockup, there will never be more than 14M BTC available. Coins that have been lost are irretrievable.
You can know count or identify the exact number of lost Bitcoin. You can rely on some on-chain analytics to estimate a number of probably lost Bitcoin. In reality, the number of lost Bitcoin can be higher or smaller than your estimation as well as ones from experts.
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HODL waves and make your estimation. I don't think we need to get exact number because the importance is it helps us to realize the scarcity of Bitcoin