A dozen means exactly twelve of something. When you buy a dozen eggs at the grocery store, you get twelve eggs in the carton. When a baker makes a dozen cookies, they make twelve cookies.

You'll also hear the term baker's dozen, which means thirteen instead of twelve. Long ago, bakers would add an extra roll or pastry to make sure they weren't accidentally shortchanging their customers. That thirteenth item became known as the “baker's dozen.”

When something happens many times, people sometimes say it happened dozens of times, meaning many groups of twelve, though in casual speech this usually just means “a lot.” But when you're counting precisely, like ordering donuts or pencils, a dozen always means exactly twelve.