Place all ingredients in a large stockpot or a 6- or 8-quart Instant Pot. In the IP, to save space, you can break up the chicken carcasses a bit. This should be easy to do with your hands. For all pots, wash the carrots, celery, and onions or leek tops and cut into large chunks. There is no need to peel.
In an IP, add water to the max fill line. (Take this limit seriously, making sure that all solid and liquid ingredients rest at or below the line, otherwise the pot won't pressurize correctly.) In a stock pot, add 8 quarts water.
Instant Pot instructions: Postition lid and set vent to sealing. Cook on manual, high pressure, for 120 minutes. (It's faster to press the down arrow, since the counter will wrap back around to 120 after it hits zero.) Pot will take quite a while to come up to pressure before the countdown begins. When cooking time ends, let pressure release naturally.
Stovetop instructions: Bring to a boil over high heat, then reduce heat and simmer gently, uncovered, for 4 hours, stopping by from time to time during the first hour of cooking to skim off and discard any foam that collects at the top of the pot.
After cooking, let stock cool to a comfortable handling temperature
Strain through a fine-mesh sieve, lined with cheesecloth if you like, into a very large bowl.
Pour into storage containers and chill. Stock is ready to use in recipes.