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?
Too many discussions already in the forum about reaching the supply limit. You can search for most of those topics through searching at the top right of the search box. Well, but to have it a summary, transactions will still happen and still will be confirmed by the miners.
will BTC price goes up instantly due to scarcity?
That's basically it, applying the law of supply and demand but not instantly.
is it the doom for Bitcoin since there is no room to mine? or maybe it will likely never reach 21 million?
Not doomed, it's unlikely that we'll still be alive when all bitcoins are mined.