It says that there will only ever be 21 million Bitcoin and the current BTC supply is almost at 19,094,150
.or maybe it will likely never reach 21 million?
Yes only 21 million, meaning that Bitcoin cannot exceed that number 21 million total supply but Bitcoin moving around (in circulation) can be lesser due to the addresses that people lost their keys.
is it the doom for Bitcoin since there is no room to mine?
No it not a doom, rather high transaction fees might be the case in my opinion since miners do not mine and get rewards any more but via transaction, Secondly I think the demand would be on the increase side as people would realize that no more Bitcoin is left to be mined, therefore people will HODL Bitcoin like a precious Gold and it will become more Scarce than it is right now.
While the early birds (early investors) like you and I will enjoy.
will BTC price goes up instantly due to scarcity?
The Bitcoin price is not going to go up drastically, it's more of a gradual process as the law of Demand and Supply takes place.
What happens when its limit reached?
No one knows until will get there but looking Bitcoin scarcity now that the mining is still in place and when it's certain that no more Bitcoin to be mined, Then that has given you a clue of what might be.