It's a bit of a miracle that all the numbers used in the system work in such a perfect way.

• 10: The Bitcoin system is designed so that, on average, a new block is mined every 10 minutes.
• 50: The initial block reward at genesis(2009) was set to 50 bitcoins. This is the reward a miner gets for solving a new block. Every roughly 4 years this number is halved.
• 2016: The mining difficulty is adjusted every 2016 blocks to keep blocks 10 minutes apart on average. If the miners are too fast, the difficulty increases, and if they're too slow, it becomes lower. This number comes from choosing roughly 2 weeks as the amount of time between difficulty adjustments. 2016 = (60 * 24 * 7 * 2 / 10).
• 210,000: The mining reward is halved after this many blocks have been mined. It's roughly 4 years assuming constant 10 minutes blocks (60 * 24 * 365 * 4 / 10 = 210,240)
• 21,000,000: This is the maximum amount of bitcoins ever to exist(Note that for some technical reasons there will be a few less sats than that though).

It was an educated guess by Satoshi Nakamoto

You can read a bit more about this here