1 is also a number, a number we chose by convention to be a base unit for all numbers. You can break down every number down to this unit.

20 is 20 1s. 1.5 is 1 and a half 1.

If we have Pi as a unit, circumference of a circle would be radius*2 of Pi units. But everything that doesn't involve Pi would be a fraction of Pi, e.g. a normal 1 is roughly 1/3 of Pi units, 314 is roughly 100 Pi units, etc. etc.