We know that 21 million Bitcoin cap is fixed rule in code that can never be changed. New bitcoins are created by miners and amount they get is cut in half every four years in halving. This process will continue until last bit of Bitcoin is mined which maybe mined in year 2140. After that no more will ever be created. Because this fixed supply makes Bitcoin rare like gold it is considered valuable. Even though supply is limited it will be enough for everyone because one Bitcoin can be split into 100 million smaller units means satoshis which can be used for smaller transactions.