Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Add the word that if there is any danger of misreading without it
Add needed words
Clarity

If there is no danger of misreading, the word that may be omitted when it introduces a subordinate clause: The value of a principle is the number of things [that] it will explain. When a sentence might be misread without that, however, include the word.