The number of BTC is fixed and it is 21M. each transaction has to be verified by Miners in order for it to be a valid transaction. Mining is very costly process and in return Miners get a reward with BTC. The difficulty of mining is getting higher and higher. My doubt is when we reach the limit of 21M, the difficulty of mining might be very costly and what if miners are no more interested in mining. Who will be there to verify transactions. Let's say however we hit the limit of 21M and then there is nothing to mine and then who is going to verify the transaction.
If miners leave mining that is fine, then difficulty goes down and it becomes more profitable for those that still mine. That's not a problem. Also the 21M limit won't be hit for many decades.