6 is one less than 7 (the number of perfection/completion/God). 6 symbolizes imperfection ( that’s why 666 is the number of Satan)
https://www.biblestudy.org/bibleref/meaning-of-numbers-in-bible/6.html
https://www.biblestudy.org/bibleref/meaning-of-numbers-in-bible/7.html