The earliest mathematical form is abstract counting, which developed from prehistoric concrete counting.
Archaeological evidence shows that when agriculture-based civilization emerged, the need to track the harvest of goods led to the use of specific tokens representing certain goods. For example, a symbol of a certain shape represents a bushel of barley. Tokens of different shapes represent a bag of grain, and so on. These concrete marks were later replaced by symbols drawn on clay boards with handwriting pens. Each kind of object still has its own symbol, and counting is just a simple repetitive symbol, which corresponds to objects one by one. For example, calculating four barrels of oil, ten bushels of barley and two pieces of clothing might look like this:
Because different quantities of wheat grains use different labels, abstracting numbers is a key step: a bowl, a container (10 bowl) and a bushel (6 containers or 60 bowls) have different labels. Therefore, if a person has 67 bowls of barley, instead of repeating the symbol of the bowl 67 times, it is better to write the number more compactly, namely bushels, utensils and bowls. For the convenience of various objects, the symbols of barley units with different sizes are abstracted from the concrete association with barley and become symbols of abstract numbers 1, 10, 60, which can be used to calculate any object. Once this abstract leap occurs, the numbers themselves can be vertical, regardless of the connection with the objects they may represent, thus laying the foundation for the birth of arithmetic. As they say, the rest is history.
For more details about this story, please see my understanding of how numbers began to exist.