How did Satoshi Nakamura come up with 21 million, for his (their?) limit to bitcoin?
Satoshi Naka
moto settled on a target time of 10 minutes per block with an initial subsidy of 50 BTC set to halve once every 4 years.
4 years / 10 minutes ≈ 210 000
50 + 25 + 12.5 + 6.25 + ... = 100
210 000 * 100 = 21 million
So, the question could be why did he chose 50. Why not 100 or any other number. 4 years too could be arguably unique. But there are several reasons why 4 years and 10 minutes fit.