There was actually a reason behind it that had to do with how bytes are divided or something like that, or maybe with the gold supply... I remember reading some theory about it, so it was not fully arbitrary, there was some thought put into it.
It's similar to the 1MB blocksize, it is what it is, but there is also thought put behind the number. Satoshi didn't simply roll a dice and choose these numbers.
It's detailed here -
https://bitcoinmagazine.com/articles/satoshis-genius-unexpected-ways-in-which-bitcoin-dodged-some-cryptographic-bullet-1382996984/Quite stunning how much thought went into every possible aspect. That's why I think it was a group that did. A bunch of heads finds many more potential holes than one.
Truth to be told I don't know about blockchain.
TLDR: I just thought 21 million supply is just a hunch, I think Satoshi knows that he/she/they will/would not live in the year of 21st century that is why it was calculated 21 million instead of 1 million supply or it is just my intuition.