What exactly is the world of S.Ex.N like? Cause the numbers reference humans, and refer to themselves as numbers, but they're also like... Humans?
Also, why aren't there any negative numbers yet?
And lastly, what's the time period of the existing humans?
S.Ex.N. is in many ways similar to an Earth environment because it essentially copies the nearest reality they originated from. As a number does its "job" by being included in counting sequences, mathematical theorems, being the quantity used to describe an arbitrary pile of pebbles, w/e, it becomes more aware of the reality describing it. Over time, the dimension the numbers find themselves in serves as basically a smudged carbon copy.
This isn't a perfect analogy or timeline either, but just something that happens to be. When Zero and Two met, it was after a majority of humans began to adopt zero as a number and mathematical object at the same time "Arabic numerals" were becoming popular. But nothing explicitly demonstrates that Zero and Two are reminiscent of humanitys cultural or stylistic trends at the time of this boom hundreds of years ago.
Humanity has a time stream that just continues forward, but in the number line they can pick and choose from any era in Earths past to find one to duplicate and be influenced by. There probably were times in the number lines past where things seemed less "modern," and it just so happens that right now they collectively feel the current trend is late 20th to early 21st century, and is just the most convenient fashion. The current time period for humans? It could be right now for all I care.
As for what they ARE, yes, they are in many ways "humans" and act and think like them, but unlike humans they are personified to represent the being with which they are. So Six for example is the epitome of everything that makes six "six" which includes her parity, prime factors, the history of how the numeral looked to other people, etc. You take all that and give it sentience, it will start resembling a human being in some ways, and then once these beings begin to search for purpose beyond the day to day hassle of semi existing, she puts on cute dresses and learns to play the cello.
These ideas - and negative numbers - will be explored more later in the series, so if you feel like these answers are kind of shitty, just know that there's a lot in the works.
