Why did Satoshi make the supply exactly 21 million. They would select 20 million or 50 million or any amount. Is there any particular reason why the amount is 21 million?
There might be no specific reason behind why satoshi created 21 million bitcoins but as far as I can guess, I think 21 was a lucky number for satoshi which made him create 21 Million bitcoins only. Perhaps 21 might be birth date of his loved ones or something like this if we think farcically. There is not always a reason behind why something like this is been numbered.
Satoshi was/is a great personality with a great mind and hence he might have encountered with a number of 21 million for some or the other reasons. Perhaps he would have calculated the mining speed of bitcoins and would have predicted the exact time or day when those 21 million bitcoins would completely be mined out so that something more exciting some in between the blockchain network.