The supply of bitcoin is finite, and already 80% of it has been mined, so only 20% remains. According to a news release by Bitcoin News as at 13 Jan, almost 16.8 million coins were in circulation remaining with only 4.2 million yet to mined to reach the total supply cap.
The coins are expected to become more valuable even as scarcity sets in as it will spur an increase in demand. The coins will become valuable and even harder to obtain once the mining of all the 21 mln has been done. Satoshi Nakamoto, the pseudonymous BTC creator, introduced the supply cap protocol so that the digital currency has some scarcity.
Bitcoin mining difficulty is intensified with the continuous decrease in mining rewards over time. Usually, after every 210,000 blocks (approximately four years), mining rewards get halved as per the Nakamoto’s protocol. So the miners now get 12.5 BTC for block mined will in the next 2 years (June 2020) receive a reduced reward of 6.25 BTC per block. A miner has to solve a complex mathematical problem to get a bitcoin fraction as the reward.
Other cryptocurrencies are not as bitcoin which is mineable. Non-mineable digital currencies such as NEM, IOTA, ripple, EOS, and Stratis are created at once so that part of the total supply is in circulation while the rest is held. Others like ether lack a market cap as they are generated when a transaction computation cycle goes on.