One of my personal assumptions is that Satoshi was orientating himself on gold. If you calculate how many standard bars could be forged from the total amount of gold on earth, you come up with around 20 - 23 million standard bars, depending on the source of information.
Of course, this is only if you add in the estimated unmined gold reserves.
To think further, if you own one Bitcoin and it exceeds the market capitalization of gold, just to give you an idea, it's like owning a 12.4 kg gold bar.
EDIT:(after numerous feedback)
Thanks for your inputs! I missed the fact that my theory only works with todays data.
https://www.gold.org/goldhub/data/how-much-goldIt was difficult for me to find a specific source for the estimated unmined gold reserves back in 2000-2008 but it will be less than today. Minus about 50,000 tons which were mined since 2005. So when we redo the calculation with my guessed data from back then, we get way less than 21 million standard bars.
I apologize for the "primitive" expression of my thoughts and also for my incorrect wording. Of course i did not meant, that Satoshi included this as a main reason, why there are 21 million.
I did not give a direct source because, as has already been noted, there are various estimates and I therefore did not consider it necessary. Very unprofessional of me!
It's an idea I've been pursuing for a while, as I see it as an interesting way of relating the amount of Bitcoin to an already proven natural resource. This could illustrate to critics (who claim Bitcoin is far too expensive and too less in circulation) that the 21 million Bitcoin could be the 21 million "standard bars", and for trading in the smaller market, the smaller units.
Just for illustration:
1 BTC as 1 standard bar of gold (12.4 kg as held by banks today)
1000 sats could then correspond to 0.124 g of gold.
This in turn shows how nicely Bitcoin can be divided up in contrast to gold. It was important for me to share this thought as it allows Bitcoin to be visualized in a natural way if someone cannot yet grasp whether it is made for this world.
