Is there a site that has a comprehensive list of fees for different sites?

IE: Coinbase charges 1% to buy or sell btc/usd
Circle charges X%, etc...

Looking for the cheapest place to buy/sell different coins

Ok,
Forgive my ignorance, but I'm new to bitcoin (love it!) and doing a lot of research.
How did Satoshi Nakamoto come up with 21 million, for his (their?) limit to bitcoin?
Because we use base 10, it seems to me that 50 million, 100, million, or 25 million would be a 'better' number to have ended up with.
Or, if you wanted to derive a mathematical number, why not double a number, to get to 16.7 million (16,777,216) or 33.5 million (33,554,432) bitcoin?
21 million bitcoin just seems like a random number.
Or is it?