The supply rule
Bitcoin's issuance schedule is encoded in the protocol and reduces the block reward through periodic halvings.
The result is a predictable supply curve that approaches, but does not exceed, 21 million bitcoin.
Halvings reduce new issuance at set block intervals, making the pace of new supply increasingly small over time. Nodes independently enforce the limit, so an attempted block that creates more than the permitted reward is rejected by the network.
Mining after issuance
When block subsidies disappear, miners will rely on transaction fees as their primary source of revenue.
The network can continue operating, but fee demand and mining economics will determine how much security the market can support.
Fees are paid by users competing to include transactions in blocks, and their level can vary with demand for onchain space. A durable fee market would need enough activity and willingness to pay to compensate miners as subsidy revenue declines.
Scarcity and availability
Not every issued bitcoin is actively available: some coins may be lost, held by long-term owners or stored in institutional products.
This difference between theoretical supply and liquid supply influences market liquidity without changing the protocol cap.
Lost keys can make coins practically inaccessible, while long-term holders may choose not to sell at current prices. Estimates of liquid supply are therefore uncertain and do not change the consensus rules that determine how many bitcoin can be issued.
What investors should watch
The cap can support a scarcity thesis, but it does not guarantee price appreciation.
Demand, custody, regulation, network security, fee markets and competing assets will remain just as important as the fixed supply.
Investors should distinguish a predictable supply schedule from a forecast about market value. Adoption and demand can change, and network security relies on miners remaining economically motivated, so both sides of the market deserve attention.