The supply is limited to 21m by lines of code. Just like everything we use controlled by software (phones, internet, cars, coffee makers, electrical grids, etc) we don’t have to understand the code to use it. Because Bitcoin is so valuable, many software developers who do understand the code watch it constantly and will let us know if anyone tries to change the code. Everyone who has moved their value to the Bitcoin network will never switch to a new code that diminishes our value by allowing more than 21m bitcoin. Because the bitcoin code is open source and widely distributed, there is no one in the world that can force Bitcoin users to run code changes they oppose. Thus, the amount of available Bitcoin will never exceed 21m.