It says that there will only ever be 21 million Bitcoin and the current BTC supply is almost at 19,094,150. What happens when its limit reached? will BTC price goes up instantly due to scarcity? is it the doom for Bitcoin since there is no room to mine? or maybe it will likely never reach 21 million?
We won't be there to witness what happens, but within out lifetimes the vast majority of BTC will be mined, and then only scraps will be left of the long time. I think this transitioning can help to stop caring about mining new coins and focus on confirming transactions and getting transaction fees instead, so I don't think that reaching the cap or getting very close to is will be a big deal. As for what happens next, as others pointed out, it depends on the demand. If something is scarce, it doesn't necessarily mean people will want it. So we don't know what will happen to the price.