I have had a similar question in mind to ask, but instead of asking why Satoshi set a cap of 21 million BTC, we could go advanced. We pretty know that the limited supply of bitcoin is it's sustainable power. If it's not limited and decentralized, it will not flourish. This is because, a group of powerful people could storm the market anytime and change the algorithm.

Can we now ask a more advanced question of why the 21 million choice. Any mathematical arrangement supporting that? I mean why not 20 million BTC which is even a round number than 21 million.