One of my favorite life realizations is that most systems of writing numbers starts with literal representations that break down around 4 or 5. That’s where your brain stops being able to differentiate distinct marks and just goes “IDK, a lot of lines, don’t ask me”. Like you can tell how many marks are here III but you can’t tell at a glance that IIIIII is 6.
This works with arabic (1, 2, 3 are cursive-ish versions of |, 二、and 三 tally marks, but then it gets weird at 4 and beyond, which evolved from “+”)
Actual tally marks |, ||, |||, |||| and then |||| with a slash to break into 5
Chinese/Japanese 一、二、三but then it gets irregular at 4, 四
Roman numerals go I, II, III, IV, irregular at four marks.
It’s really cool how widespread and universal that is, and what it tells us about the human brain and how it counts and thinks of things as a group
(Also, a few languages have systems of counting that tend to go “one, two, many”: https://numberwarrior.wordpress.com/2010/07/30/is-one-two-many-a-myth/ )
*Edit: ALSO from that link above, that might have something to do with why secondary counting systems (first, second, third, fourth) are most irregular for the first couple of numbers but regular after that (fifteenth, tenth) in a lot of languages