The discoveries of the tombs and mummies must have been breathtaking for those lucky few that found them.

Question

"that" or "who"?

Answer 1

What is it that you are supposed to do? I'd ask the question "What must have been breathtaking? answer = the discoveries

Sra

Answer 2

or are you supposed to add either "that" or "who?" Then "for the few who found..."

Sra

Answer 3

The discoveries of the tombs and mummies must have been breathtaking for those lucky few who found them.

In this sentence, "who" is referring to "those lucky few [people]..."

We use "that" to refer to things, "who" to refer to people.