He has come up with that number for the conclusion of making it scarce. Just like what the content of his email to Mike Hearn that says this;
From: Satoshi Nakamoto <
satoshin@gmx.com>
Date: Mon, Jan 10, 2011 at 4:34 PM
To: Mike Hearn <
mike@plan99.net>
Subject: Re: More BitCoin questions
[...]
I fudged it to 364.58333 days/year. The halving of 50 BTC to 25 BTC is after 210000 blocks or around 3.9954 years, which is approximate anyway based on the retargeting mechanism's best effort.
I thought about 100 BTC and 42 million, but 42 million seemed high.
I wanted typical amounts to be in a familiar range. If you're tossing around 100000 units, it doesn't feel scarce. The brain is better able to work with numbers from 0.01 to 1000.
If it gets really big, the decimal can move two places and cents become the new coins.