If he really set the limit to 21 billion, people will ask "why not 21 million or 21 thousand"...
Similarly, you could ask why the block reward is 50 btc at first instead of 64 or 1...
IMHO, there is no particular reason when Satoshi decided those magic numbers.
Everything about bitcoin was chosen very carefully by satoshi. I'm convinced of that. The more I look into it, the more I realize his/her/their genius and foresight, I am struggling with the number of coins though. Anyone care to postulate a good reason for 21 mil?
I just found this old thread accidentally. It may provide some insights for you.
https://bitcointalksearch.org/topic/bitcoin-protocol-magic-numbers-and-reason-263750I suspect that he choose 21 million, rather than billion or trillion, for scarcity reasons. Millions is not that much these days but trillions is still a shitload, as far as early adoption goes, saying there is only 21 million, as opposed to 21 trillion, sounds a hell of a lot more enticing.
IMO I also don't think the unit of btc matters so much. I think everyone here focuses on the btc unit too much (e.g I have 10 btc, this thing costs 2 btc). Btc is a hedge against inflation and protection from government/banks. In my eyes, its much better to quote how much fiat value one has in bitcoin, not just the number of bitcoin. If we only ever talked about the fiat value, it would never be that confusing, and it would sound awesome too. For example, "I bought $1000 worth of bitcoin in 2011 and now it's worth $2.3 million" as opposed to "I bought 1000 bitcoin in 2011, I'm rich man." "yeah, cool man, how much are a thousand bitcoin?". No point using numbers of bitcoin when everyone is going to convert it to fiat for perspective anyway.
Same goes for gold. In my country there is a lottery where you can win gold. They don't say you'll win 10 ounces (boring) they say you'll win $10,000. Just makes more sense and sounds impressive.
I say never use btc amounts (or price things in btc) when talking to people, use the fiat amounts, that way decimal places don't matter.
Or maybe I'm just an idiot, I don't know.