Add the chicken carcasses, onion, carrots, celery, apple cider vinegar (if using), salt, peppercorns, and bay leaves to a 6 or 8-quart Instant Pot. Stir well.
Add enough water to cover the bones (about 8-10 cups). Do not exceed the max fill line marked on the inner portion of the pot.
Place the lid on the Instant Pot and set the valve to “sealing.”
Cook on manual pressure for 2 hours (120 minutes; or use the “Soup” mode if available on your Instant Pot model).
Once the cook time is done, you may either do a quick pressure release by switching the valve to “venting” OR you can do a natural pressure release by allowing the pot to release pressure naturally. I prefer the natural release as the broth is more clear (versus cloudy) and I find it is more likely to gel. You will know the pressure has been fully released when the pin drops on the lid. Then, you can safely open the lid away from your face.
Optional: If using parsley, add it to the hot broth and allow it to rest for 10-15 minutes before straining.
Fit a large bowl with a fine mesh or stainless strainer. Ladle in the broth in so that the strainer catches the vegetables and bones, while the chicken broth collects into the bowl below.
Allow the broth to cool completely before tightly wrapping the bowl of broth OR transferring to a glass canning jars.
If desired, place the broth in the fridge and allow to chill for at least 6 hours, so the fat solidifies at the top and can be removed. Alternatively, you can use the hot broth with the fat in it (or store it with a fat cap), as it’s just a matter of preference!
Transfer the broth to canning jars or freezer containers for storage. Keep in the refrigerator for 3-5 days, or in the freezer for up to 6 months.