My ‘real’ opinion is that Satoshi picked the starting mining reward at 50 BTC and 210,000 blocks/halving, and ~21 million BTC is the result. It could be that Satoshi picked “a value for blocks/halving such that you get a similar looking number for total supply.”

The 21 gun thing predates the US, but I’m told that the “1776” interpretation is a common interpretation meme given to it in the military. 🤷‍♂️