And the world number is not fixed, I don't remember exactly, but I can search somewhere on the web, about when bitcoin launched, how many people there are in the world, and the present, the future.
So this calculation doesn't make much sense to me, because I also know about decentralization but most of the bitcoins are kept in the pockets of whales so there's no need to think about everyone getting an equal share bitcoin.
A big assumption is all people on the world will invest in Bitcoin, to make this calculated figure. This will never happen but this big assumption and the calculations give us a raw view on what will happen for each person if all world people will spend money to buy bitcoin.
Fortunately, because many people have yet invested in Bitcoin, we are early birds and can take advantage of it and accumulate Bitcoin with cheaper price than when more people will join this market.