Bitcoin Can Be "Mined" Only 20 Percent?
Since the beginning of its creation, the amount of Bitcoin is limited to only 21 million pieces of stock. In mid-January 2018, there were 16.8 million Bitcoin already "mined".

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That amount is 80 percent of the total Bitcoin that can be generated. That is, now there are only 20 percent or as much as 4.2 million Bitcoin that can still be "mined" through the mining process.
Mining is a collective process that utilizes the services of minerals Bitcoin computers around the world, to process transactions on the Bitcoin (Blockchain) network.
This mining process requires a large computer power. As a reward, the Bitcoin system provides a number of new Bitcoin pieces for each new recording block added to the blockchain.
The number of newly produced Bitcoin pieces reaches 12.5 pieces per block on July 9, 2016. However, the figure will be reduced by half, for every 210,000 new blocks added, or approximately every four years.
In other words, by 2020, it is likely that the number of newly generated Bitcoins will only be as much as 6.25 pieces per block.
When it first "milled" the Bitcoin block in 2009, the creator of Bitcoin Satoshi Nakamoto still got a new Bitcoin of 50 pieces
As the number of new pieces generated decreases further, the maximum number of 21 Million Bitcoin is expected to be exhausted "mined" by the year 2140.
Different from Bitcoin, Ethereum, one of the other popular cryptocurrencies, does not impose limitations on virtual money that can be generated.