Numbers are about as ancient as we humans have been sapient in any form similar to ours, it is doubtful that neanderthals didnt have some concept of it considering they used clothing and more hence had atleast some simple understanding of numbers. It is then equally doubtful that our conpeople wont have some understanding of numbers aswell.
The simplest form is Unary system, 1 line or something means 1 and 2 lines means 2 and so on. That system is very simple to understand and is used in modern terms through the way of I+1=II, IIII+1=
So people start choosing bases for their math and ways to represent it in a more coherent fashion that doesnt require counting beyond 5-7 in the individual symbols themselves. But the question becomes then "Which base do I choose?"
Well I would say there is one of two reasons any given base is really picked by people, both really does go back to "Convienience" in some sense.
Biological Reason, This mostly refers to that the base is choosen purely from what is aviable by the people, 10 fingers for example. But traces of things have given us reasons to thinkt aht europe used to have base 8 prior to base 10s entrance, mostly because 9 has often a name that coincide with "new" which might not be a coincidence on its own. But why 8? Same reason though there are 8 SPACES between fingers on human hands. Base 20 is the same, 10 fingers and 10 toes. This is an easy way, natural and understandable for any creature to pick.
Mathematical Convinience, this one is also popular in ancient civilizations, they werent nearly as dumb as we wanted to make them. 12 and 60 are common in this class but exacly waht do I mean with it? Mathematical Convinience is mostly that from a mathematical point of view it is considerbly easier dealing with in fractions decimal system and all. For example in a civilization where decimal system has yet to be developed a base with alot of divisions in it is alot easier dealing with than one with few. Why? if I got base 60 and divide a cake by 3 I know it should be (0 . 20 , 0) parts in each piece. That is of course in a decimal system, but even babylonians used it well for division, 1/13=7/91~7/90=7*40/3600=7*40/60², Sure its an approximation buit its 1% off and most things cant be solved exacly. Having a more rich divisionable base makes approximations easier because its easier to reach a good appriximation with a finite decimal string or division.
12, 24, 48 and 60 all have one thing in common, they are highly composite numbers. Meaning they got more ways of being divided with no rest than any other number prior to them. This is the reason why 60 for example was picked by some, 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60. Thats a rather immense amount of possible divisions, 12 even if its small got a good amount of divisors, 1, 2, 3, 4, 6 and 12.
If we include all possible finite decimal expansion up to the base number itself the amount grows even more. This is why many prime numbers in the base number itself is so valueble.
This is the reason these numbers have been loved by ancient people and why certain numbers come time and time again in various forms of bases or subbases while others are not that prone (but not impossible).
An example that is often a nail in my eye is many seem to take base 16. Sure it works well in our digital age of computers but for people when the choice is acctually made and not just used base 16 is horrible, 1 it doesnt have any natural representation on our own bodies (though of course I dont know how your species is, erhaps 8 fingers per hand? or 4 fingers and toes?) and mathematicly it is a horrific number to pick, why? It only got 1 prime number namely 2 in it, 2^4=16 meaning it can only properly deal with divisions of power two as finite fractions or eliminations in the number itself. While it is true all decimal expansions that are finite are considerbly less than those with inifnite expansion why would any people really want to limit themselves more?
Base 16 therefor in most cases will be one that is reserved to binary computer ages or some species where it occures naturally on their body as it possesses no value mathematicly.
I could go on forever with various bases but I shant I merely took 16 as an example.
I'll post about how numbers are represented later on if people are interested