On January 10, 2011, Satoshi explained to Mike Hearn the logic he used in determining the emission rate and the total number of coins:
It turns out to be 10 minutes per block: 21,000,000 / (50 BTC 24 hours 365 days 4 years 2) = 5.99 blocks/hour.
I specified 364.58333 days in a year. The halving from 50 to 25 BTC will occur after 210,000 blocks or approximately 3.9954 years, which is in any case an approximate calculation based on the best attempts of the difficulty adjustment mechanism.
I considered starting the emission with 100 BTC per block and reaching 42 million in circulation, but 42 million seemed too large a number to me. |I wanted typical amounts to be in a familiar range. If you throw around about 100,000 units, it doesn't feel like something rare. The brain works better with numbers from 0.01 to 1000. |If the amount is very large, the decimal point might move two places, and cents could be used as whole coins were used previously.