Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, garlic, and seasonings in a 16- to 20-quart stockpot. Add 7 quarts of water and bring to a boil. Simmer, uncovered, for 4 hours. Strain the entire contents of the pot through a colander and discard the solids.
Note: make sure water covers all ingredients. Add more water as needed.
Chill the stock overnight.
The next day, remove the surface fat floating on top. Use immediately or pack in containers and freeze for up to 3 months.