I wonder why only 21 million. . .
A block reward of 50 BTC per was chosen for a starting amount. Then a decision was made to cut the block reward in half after every 210,000 blocks. If you do the math you'll see that the total mined amount happens to work out to a bit less than 21,000,000 by the time the block reward drops below 0.00000001.I don't know for sure, but I doubt that there was any specific intention to choose the number 21 million as an upper limit. More likely 50 seemed like a reasonable amount to get things started, and then about 4 years was chosen to allow people to adjust to the new block reward before cutting it in half again.
The number 5,000,000,000 could have been chosen as a block reward, and the end amount would have been a bit less than 2,100,000,000,000,000. But 5 billion as a block reward would have been a large number to work with on a regular basis until the currency caught on. Perhaps if we just move the decimal over 8 places to the left and give that amount a name (sort of like a $100 bill being called a Benjamin) we could work with that new amount until we need to split it up more. Here's an idea. We can move the decimal 8 places to the right and call it a bitcoin. Then as we need to split it up more we can come up with new names for the smaller denominations. (perhaps using SI? milli-bitcoin, micro-bitcoin) When we get all the way down to the original unit as it is stored in the blockchain per the protocol, we can call it a Satoshi or something like that.
Note: the italicized is meant to be humorous. Unless I mis-typed an extra zero somewhere, this is the way it actually works.
