Make the soup: Add the broth and 4 cups water to a large soup pot and bring to a simmer over medium-high heat. Sprinkle the chicken with salt. Submerge the chicken pieces in the broth. When the broth starts to boil, reduce the heat to a simmer. Cook until the breast and tender start to separate, about 15 minutes, then remove with tongs or a large strainer. Continue simmering the leg pieces until the meat pulls away from the bone, about 10 more minutes. Remove the legs from the pot and set aside. When the meat is cool enough to handle, remove the skin and shred the meat by hand.
Add the carrots and celery stalks (reserve the leaves) to the soup and simmer until tender, about 8 minutes. Stir in the shredded chicken until just heated through, about 3 minutes, then remove from the heat. Season with salt, stir in the celery leaves and cool to room temperature.
Divide the soup into three 4-cup servings and freeze in containers for up to 1 month.
To reheat: Thaw a container of soup and bring to a low boil in a medium saucepan. Add the desired combo mix-ins and cook for a just a few minutes to heat through. Serve.
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