21 Million names Bitcoin’s familiar issuance cap. Rounding makes the sum of scheduled maximum subsidies slightly less than 21 million BTC; this is not the amount actually available for sale.
The initial 50 BTC halves every 210,000 mainnet blocks, rounding down to whole satoshis. Summing the maxima gives 20,999,999.9769 BTC, including the scheduled Genesis Block subsidy. Its 50 BTC cannot be spent; unclaimed subsidies and other destroyed coins further reduce availability. MAX_MONEY checks amount ranges but does not itself create the issuance schedule. [Bitcoin Core v29.0: subsidy and validation]
A Full Node checks that the Coinbase Transaction does not exceed Block Subsidy plus permitted fees. More hashrate cannot force excess issuance on nodes enforcing this rule. Software allowing more issuance does not rewrite existing nodes; adopting it means incompatible consensus rules. [Bitcoin Core v29.0: subsidy and validation]
The chain may not reveal a lost key; an old unmoved UTXO is not proof of loss. Circulating-supply estimates therefore depend on methodology. Smaller units do not create more BTC; the cap alone guarantees neither purchasing power nor future demand. [Bitcoin Core v29.0: subsidy and validation]
A geometric issuance schedule
Each subsidy era permits half as many satoshis per block as the previous era, for 210,000 blocks. Integer rounding eventually reduces the subsidy to zero, leaving total issuance slightly below exactly 21 million BTC.
For the clearest picture, read this entry together with Block Subsidy, Halving, Bitcoin, Full Node, Consensus rules. The reverse links also lead from Bitcoin, Block Subsidy, Halving, Satoshi.
01Where is the 21-million limit enforced?+
Every validating node checks the permitted block subsidy. The limit is therefore an independently verified rule, not a promise held by a company or miner.
02Could users vote to create more than 21 million bitcoin?+
Anyone can publish incompatible software, but existing nodes would reject its excess issuance. Changing the monetary rule would require users to voluntarily adopt a different network.