A passphrase is a password made from words. More independent random choices make it harder to guess; a longer familiar quotation can remain predictable. The useful question is how the words were selected, not just how many letters appear.

Entropy describes uncertainty in that selection process. For independent, equally likely choices, adding a word multiplies the possible combinations by the size of the word list. Human themes and repeated patterns do not meet that assumption. There is no universal word count that establishes adequate protection for every vault.