The numbers we all use (1, 2, 3, 4, etc.) are known as "arabic" numbers to distinguish them from the "Roman Numerals" (I, II, III, IV, V, VI, etc). Actually the arabs popularized these numbers but they were originally used by the early phonecian traders to count and keep track of their trading accounts.
Have you ever thought why .... 1 means "one", and 2 means "two"? The roman numerals are easy to understand but what was the logic behind the phonecian numbers?
It's all about angles !
It's the number of angles. If one writes the numbers down (see below) on a piece of paper in their older forms, one quickly sees why. I have marked the angles with "o"s.
No 1 has one angle.
No 2 has two angles.
No 3 has three angles. etc.
and "O" has no angles