"A dozen" means 12. Certain things come in containers of 12, like eggs and doughnuts:

Do you want a dozen or a half-dozen?

We also use "dozen" to talk about numbers that are between 12 and 100. We use "dozens" instead of "tens":

I've been there dozens of times.

There were a few dozen people there.

"A couple dozen" means about 20 to 30.