When Bitcoin reaches its 21 million coins limit, no more new Bitcoins will be mined, and the only way for the supply of Bitcoins to increase would be for existing holders to sell their coins to new buyers.
This event is known as the "Bitcoin halving," which happens roughly every four years, and it's designed to control the inflation rate of Bitcoin. The current block reward for miners is 6.25 BTC, which means that every time a block is mined, the miner receives 6.25 BTC as a reward.
However, when the 21 millionth Bitcoin is mined, the block reward will become zero, and miners will only receive transaction fees as their reward for verifying transactions.
As the number of Bitcoins in circulation becomes increasingly scarce, the demand for them may increase, leading to higher prices. However, it's also possible that the demand for Bitcoin could decrease, causing its value to fall.
Overall, what will happen when Bitcoin reaches its 21 million coins limit is difficult to predict as it depends on various factors, including market demand, technological developments, and regulatory changes.