1. Specifically in a group or quantity of 12 (which constitutes a dozen). You can't just get one cupcake at that place—they only sell them by the dozen, like eggs.

2. In large numbers. Despite the using the word "dozen," the phrase is not limited to quantities of 12 and can be written in the plural ("dozens"). If we institute new rules by the dozen, we'll never be able to police them all at once. We're going to need to get bagels by the dozens for the company-wide breakfast meeting.