by the dozens

by the dozen(s)

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.

by the dozens

Fig. many; by some fairly large, indefinite number. (Similar to but implying less than hundreds. Compare this with by the dozen.) Just then people began showing up by the dozens. I baked cookies and pies by the dozens for the charity bake sale.
See also: dozen
References in classic literature ?
In the midst of them, the hangman, ever busy and ever worse than useless, was in constant requisition; now, stringing up long rows of miscellaneous criminals; now, hanging a housebreaker on Saturday who had been taken on Tuesday; now, burning people in the hand at Newgate by the dozen, and now burning pamphlets at the door of Westminster Hall; to-day, taking the life of an atrocious murderer, and to-morrow of a wretched pilferer who had robbed a farmer's boy of sixpence.