To stuff your turkey, you'll need 4 cups unbaked stuffing for a 12-pound bird. Stuff it right before roasting and bake remaining stuffing in a 2-quart baking dish.
Preheat oven to 400 degrees, with racks in middle and lower thirds. On two rimmed baking sheets, arrange bread in a single layer. Toast until dry and pale golden brown, 10 to 12 minutes. Let cool.
Meanwhile, lightly butter a 9-by-13-inch baking dish. In a large skillet, melt butter over medium. Add onion, celery, and garlic and cook, stirring frequently, until onion and celery are softened, about 7 minutes. Cook, stirring, 1 minute. Transfer to a large bowl and season with salt and pepper.
Add parsley, eggs, and bread; stir to combine. Add broth in 2 additions, stirring until absorbed after each addition. Season generously with salt and pepper and transfer stuffing to dish. Bake on middle rack until deep golden brown on top, 25 to 30 minutes. Let sit 5 minutes before serving.
Make-ahead: Toast the bread up to 2 days ahead and store in airtight containers or zip-top bags.