Why was 21 million picked as the number of bitcoins to be created?
Why did Satoshi pick 21 million as the number of bitcoins to be created? What is the significance of that number?
I added the finance tag since this question is asking about not just the math behind how 21 million blocks was created, but the reasoning behind all that math from an economic perspective (assuming there was one)
Here's a mathematical explanation:
Calculate the number of blocks per 4 year cycle:
6 blocks per hour * 24 hours per day * 365 days per year * 4 years per cycle = 210,240 ~= 210,000
Sum all the block reward sizes:
50 + 25 + 12.5 + 6.25 + 3.125 + ... = 100
Multiply the two:
210,000 * 100 = 21 million.
Economically, because the currency is effectively infinitely divisible, then the precise amount doesn't matter, as long as the limit remains fixed.
Good explanation, but you're just postponing the answer. Why 4 years per cycle? Why was 4 years per cycle picked as the number for years per cycle?
If you check my original answer, you'll see I ended with "but I don't know the economics behind it". I see that has since been edited - not by me.
Interesting. Does this mean that slightly more than 21 million bitcoins will be created?
No, slightly less than 21 million, because of the rounding error on the last halvings.
I think we best conclude that nobody knows why Satoshi chose for 21 million or 4 years per cycle. But I would not recommend accepting this answer as correct. In fact it only answers "What's the maximum amount of bitcoins that can exist?".
The American presidency could be the 4 year schedule