I am an unreliable cook. That onion looks like it would be about 1 cup. Those ribs of celery are probably more than a cup, but why wrap up a half-rib of celery? Just throw it in. I substitute ingredients. I use up leftovers. I become enamored of a certain flavor or spice and throw it in. So goes my stuffing. One year, I think people actually got a buzz from the stuffing. A little Grand Marnier sounded like a good thing. A lot of Grand Marnier sounded even better. Oops. The recipe I usually work from is the first one listed under stuffings and forecemeat in an old Joy of Cooking I've had for decades.
There are lots of optional ingredients and choices. I always use garlic, onion, celery, and lots of fresh parsley. This year the nut of choice was pine nuts, which I toasted first.
An absolute essential is the apple sausage from Taylor's Old Fashioned Meats in Sierra Madre. As for the bread, I've used fancy bakery bread, croutons from supermarket bakery, Trader Joe's boxed stuffing mix (once) --these boxed things are always weirdly seasoned, in my opinion. For a couple of years, I used La Brea Bakery olive bread. This year we went gluten free, rather than make a special version of the stuffing for my daughter who cannot eat gluten. I used Whole Foods white gluten-free sandwich bread, drying it out for several hours on the kitchen counter. After adding a liberal amount of chicken broth, it was perfect. I had to cook it on the grill because the oven was stuffed with turkey and almost burned the bottom. When I warned my guests that they might not want to scoop too deeply into the pan, there was a chorus of, "Oh, I love the crusty burned part!" I guess I'll always make the stuffing on the grill from now on. It just might have been the best stuffing ever.