Fewer/less than a dozen (people)

"A Dozen" literally means "12". They are interchangeable in every context, even if it sound odd for phrases we are used to. The movie, "The Dirty 12"? "Ocean's Dozen"?

12 is a counting number, an integer. Fewer than 12. Fewer than a dozen. To be precisely correct. Even so, there will be time 'less than' sounds ok. "I'm less than a dozen miles from home, please turn the lights on."