Two People with Same Number of Hairs on Head

Problem
Why must there certainly be at least $2$ people in the world with exactly the same number of hairs on their head?

Solution
It can be assumed that the most hairs a person has on their head may be of the order of a couple of hundreds of thousands.

The number of people in the world is (as of $2020$) approaching $7$ billion.

The result follows by the Pigeonhole Principle.

So, select individuals arbitrarily, and count the hairs on their head.

Sooner or later, you will pick someone who has the same number of hairs that you have already selected.

This will happen by the time you have selected a few hundred thousand people, at the very latest.

That is, way before you have counted everybody in the world.