The rules of Bitcoin fix the total supply: there will never be more than 21 million bitcoin. To be exact, a little less, about 20,999,999.98.
A schedule written in advance
New bitcoin are created as a reward to whoever adds the next block to the chain. In 2009 that reward was 50 bitcoin per block. Every 210,000 blocks, roughly every four years, it is cut in half. This is the halving.
50, then 25, 12.5, 6.25, and 3.125 since April 2024. The reward keeps halving until it becomes too small to divide, around the year 2140. After that, no new bitcoin will ever be created.
Who could change it?
Nobody alone. The limit is enforced by every node. A miner who paid themselves one extra coin would see their block rejected by the whole network. Changing the limit would require nearly every user to install software that breaks the rule they chose Bitcoin for.
▶ SEE IT IN THE MACHINESUPPLY shows how many of the 21 million already exist, live, and HALVING how many blocks remain before the next cut.