Marcie finds a djinn, frees it and gets a wish. She wants to get herself a wedding gift, a trousseau (apparently, a hope chest is intended although it could mean other things)--a term the djinn is unfamiliar--so she rattles of a number of items she'd like inside. She gives up and says, oh, just get me a dozen of everything and enlarge the house so it fits.
You can probably guess that she got a dozen of everything in the world. You may not have guessed she'd also have a dozen copies of her fiancé....
* * *
To back up what I said earlier about Jonathan Franzen's negation of the cliché, I offer up this story. Bradley uses clichés to characterize Marcie and her aunt. However, as a few clichés may be dated, I'll go out on a limb and guess what they're meant to indicate.
"Aunt Hepsibah was, as the vulgar expression puts it, rolling in dough."Marcie is at least middle class with aspirations of higher. She is "a normal child of her generation" and went to the movies often, so her parents weren't poor. She's aware of the informal cliché, "rolling in dough", but feels she should be above it and call it "vulgar."
She is at work (likely clothed) and says the cliché "without a rag to wear" (in use since at least the late 1800s and used by writers like Penelope Fitzgerald, Booker Prize winner, and Garry Wills, Pulitzer and National Book Critics Circle winner). Moreover, Marcie is probably not arriving at work wearing rags, either. Again, we're looking at middle class with higher aspiration. She also uses the French term, chichi, so once more she's preoccupied with matters of taste and using terminology from other cultures, which backs up my earlier speculations. That she uses these phrases shows she is somewhat fashionable (which you may take as positive or negative).
Finally, she orders a "dozen of everything". That's probably a middle class trait to stock up on stuff that you can, hoarding for a later date that will probably never come. Why would you need a dozen of everything? If the story has a deeper meaning, that's probably it. Why ask for more than what you've got?