A miner who finds a block earns two things through the coinbase transaction: the block subsidy, newly created bitcoin, and the fees of every transaction included.
Halving every 210,000 blocks
The subsidy started at 50 BTC in 2009. Every 210,000 blocks, about four years, it is cut in half: 25 in 2012, 12.5 in 2016, 6.25 in 2020, 3.125 in 2024. The next halving brings it to 1.5625 BTC, around 2028.
Adding up this series gives just under 21 million bitcoin. The last fraction of a satoshi will be issued around the year 2140. After that, miners will be paid by fees alone.
A miner cannot pay themselves more
If a coinbase claims even one satoshi more than the subsidy plus the fees, every node rejects the block, and the miner has burned the electricity for nothing. The schedule is enforced by verification, not by trust in miners.
▶ SEE IT IN THE MACHINEHALVING is a stepped ruler of seven steps, each half the one before; its roller follows the real count of blocks to the next halving.