What happens when its limit reached?
When all the bitcoins are mined, which will happen in about 118 years, the miners' income will come solely from transaction fees.
will BTC price goes up instantly due to scarcity?
It can't go up instantly, because:
- Coins are brought into circulation gradually.
- Demand is another factor, irrelevant of supply, which affects the price.
is it the doom for Bitcoin since there is no room to mine?
As said, the main income will sooner or later come from transaction fees. This will happen much sooner than 2140.
or maybe it will likely never reach 21 million?
Who knows what can happen within the next 118 years.