“Why was the human being created as an individual? To teach you that one who destroys a single human life is as though he has destroyed an entire world. And one who saves a single human life is as though he has saved an entire world.”
Mishnah, Sanhedrin 4:5.

Simple arithmetic does not apply when dealing with human lives. Each individual equals an entire world.

The Talmud provides a practical application:

A caravan traveling on the road is accosted by strangers who tell them, “Give us one of you and we will kill him, and if you refuse, we will kill all of you.”

Ignoring the math, the Talmud rules: Even if all of them will be killed, they must not hand over a single soul.

Why? Because each human life contains at its core the very essence of G‑d. As G‑d is infinite, so the value of a human life.

And one infinity is not any less than a hundred, a thousand, or even eight billion infinities.

Reshimot 123. For further elucidation, see How Each of Us Contains All of Us.