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.
That was really a good read. So 21 million limit is not just an arbitrary number. Satoshi was a mastermind and he wouldn't choose just a random number when he could potentially expect Bitcoin to workout the way it is now. As the article said, Satoshi chose this number based on mathematics as the largest integer that can be stored in a standard integer on a computer. I don't know whether this is true or not but then how would the other crypto currencies with larger supply than BTC would workout ?