As of September 2019, there are 17 active bitcoin mining pools. Bitcoin mining pools are a way for Bitcoin miners to pool their resources together and share their hashing power while splitting the reward equally according to the amount of shares they contributed to solving a block.
A “share” is awarded to members of the Bitcoin mining pool who present a valid partial proof-of-work. Shares are a way of representing proof-of-work quantity rather than quality.
It is possible for a single miner to mine a block solo – although the odds of this happening are extremely low since it requires an enormous amount of computational power. By joining a mining pool, miners increase their chances of successfully solving a block and receiving a reward.
Mining pools have become increasingly popular as the difficulty of mining Bitcoin has increased. As the name suggests, solo miners mine alone – i.e. without joining a pool.
The rewards from solo mining are much higher but the chances of successfully solving a block are also significantly lower. When miners join a pool, they receive smaller rewards more frequently, which makes it easier to predict earnings and provides stability for miners who might otherwise be constantly moving from one unsuccessful solo mining attempt to another.
The first known bitcoin mining pool was called “Slush’s pool”, founded by Satoshi Labs CEO Marek Palatinus (aka Slush). Slush’s pool has since grown to become one of the largest and most well-known mining pools in existence, with over 1% of the network’s hashrate under its control.
Other notable mining pools include: BTCC, F2Pool, Antpool, BitFury, and Coinbase’s GDAX. All of these pools are reputable and have been in operation for many years.
In conclusion, there are currently 17 active bitcoin mining pools. This number has likely grown since September 2019 as the difficulty of mining Bitcoin has continued to increase and more miners have joined pools in order to increase their chances of successfully solving a block and receiving a reward.