Protocol:
- Each block creates new coins (the "block reward") in its "coinbase transaction".
- The block reward was initially at 50 BTC per block.
- Every 210,000 blocks (~4 years), the reward is halved.
- This halving produces a geometric series that converges to a maximum of 21 million BTC.

Enforcement:
- Bitcoin nodes run the protocol code.
- The code defines the block reward formula and enforces it.
- A miner who tries to claim more than the allowed reward will simply produce an invalid block.
- Other nodes reject that block, and the miner gets nothing.

Note:
- A subgroup of nodes can choose to set a higher limit, producing a hard fork, resulting in a new network, with the same addresses and balances. This has already happened at least twice. However, the original network offers a "harder" asset (due to its lower limit) and over time wins in terms of stored value and market interest. Holders will tend to sell the forked asset.