You can freeze the stock in jars (leave space on the top for expansion), freezer bags, containers that can be frozen, and even ice cube trays.You don't have to add a complete roasted chicken. You can add what's left over from using the meat and just place that in the pot. Even if you only have the bones left, that's enough to make stock.
Instructions
In a large saucepan, slow cooker, or Instant Pot, add the roasted chicken, or left overs from a roasted chicken, and 4 to 6 quarts of water. Optionally, you can also add cut carrots (which make the stock sweet), leeks, celery, herbs, kosher or sea salt (if you want it pre-seasoned). If you are using a roasted chicken from the store (pre-roasted, usually rotisserie style, and ready to eat) and it has extra seasonings, take the skin off to avoid over-seasoning the stock. I suggest purchasing the most natural version of the store-bought roast to avoid additives and unwanted seasonings.
Let it simmer for at least 3 hours, and up to 6 hours.
Turn off the heat, cool the stock, and then strain the liquid using a strainer.
Now use the chicken stock for a recipe, or freeze the stock. I usually freeze the stock in glass jars, but you can use any kind of container that can be placed in the freezer. When I use glass jards, I leave some room at the top for the liquid to expand when it freezes. I also freeze different portion sizes to make it easier to defrost and allows for different size portions, such as one or two bowls of soup at a time.
You can defrost the frozen stock containers in a bath of cool water (about 30 minutes), or thaw in the refrigerator or at room temperature. For a thawing emergency, and with the top off, place the jar in a microwave on a defrost setting for several minutes.