A one gal plastic bag in my freezer fully stuffed with vegetable odds and ends will yield about 2-3 quarts stock when processed with my 6L pressure cooker. I will add seasoning and fill the water to the max line, then strain while it's hot after the cook.
Just a quick note: a pressure cooker is going to be hot enough to destroy some aromatic flavor compounds found in many ingredients, like onion and garlic. It depends on the types of veg you use for stock, but I've had better luck leaving it in a pot for several hours on the lowest heat—it's almost an infusion as opposed to a traditional "stock", but I find that I get way more nuanced flavor out of it that way.
