Cut up 2 (3-pound) chickens, or use any combination of thighs, drumsticks, or breasts to make this chicken stock. Use a pan large enough to roast the chicken and all the vegetables in a single layer. If the pan is too small, the chicken won't brown properly.
Preheat oven to 400°.
Arrange the first 4 ingredients in the bottom of a broiler or roasting pan, and top with chicken pieces. Bake at 400° for 1 1/2 hours, turning the chicken once every 30 minutes (chicken and vegetables should be very brown).
Place the peppercorns, parsley, thyme, and bay leaves in an 8-quart stockpot. Remove vegetables and chicken from broiler pan, and place in stockpot. Carefully discard the drippings from broiler pan, leaving browned bits. Place broiler pan on stovetop; add 4 cups of water. Bring to a boil over medium-high heat. Reduce heat; simmer 10 minutes, scraping the bottom to loosen the browned bits.
Pour contents of broiler pan into stockpot. Add remaining 12 cups of water, and bring to a boil over medium-high heat. Reduce heat, and simmer 1 1/2 hours.
Strain stock through a fine sieve into a large bowl. Reserve chicken for another use; discard remaining solids. Cover and chill stock for 8 hours. Skim solidified fat from surface of broth, and discard.