Question

Why we have only two ears. Why we don't have more?

Answer 1

We don't have more than two ears because during evolution we never needed more than two.

Every extra body part requires energy, in a survival situation extraneous body parts become a liability food wise, and something else for predators to get hold of.
Extra body parts only evolve if they improve an organisms chance of surviving to reproduce.
For instance, Anableps Anableps has evolved two distinct and separate eyes, one pointing up, the other down, the increased field of vision helps the animal to navigate, locate food and evade predators.
We don't need four eyes, because unlike A. Anableps, we have the ability to turn our heads.

Similar is the reason why we don't have more than two ears