There are currently close to 4.3 million Bitcoins left that aren’t in circulation yet. With only 21 million Bitcoins that will ever exist, this means that there are about 16.7 million Bitcoins currently available. Out of those 16.7 million, it’s estimated that 30% of those may be lost forever as a result of things like hard drive crashes and misplaced private keys.
The remaining Bitcoins not in circulation are in a pool dedicated to rewarding miners for maintaining the integrity of the network. As miners validate transactions and create new blocks, they receive the remaining Bitcoins from this pool as a reward. The Bitcoin source code outlines how the mining rewards should be distributed and when these distributions occur.
The reward for mining each block started at 50 Bitcoins and has since “halved” twice. The current reward sits at 12.5 Bitcoins per block.
The mining reward halving occurs every 210,000 blocks. With blocks taking about 10 minutes on average to mine, halvings occur about every 4 years.