Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, dill, garlic, salt and pepper in a 16- to 20-quart stockpot. Add 7 quarts of water and bring to a boil over high heat. Skim any foam that comes to the surface. Lower the heat and simmer, uncovered, for 4 hours.
Strain the entire contents of the pot through a colander. You can discard solids if desired. (I save the chicken, cool, pick the meat from the bones and use immediately, or freeze.) Pour stock into containers and refrigerate. When the stock is cold, remove the surface fat and refrigerate or freeze.
Recipe by Not Just Sunday Dinner at https://notjustsundaydinner.com/thanksgiving-dressing/