Every Goodie Bag has a 100% chance to grant two random items out of a prize pool and a 50% chance to grant a third item from the same prize pool. Also, a unique item and the empty Trick-or-Treat Bag are guaranteed to be granted upon opening. So you are guaranteed to get 2 unique items and 2 random pool items, with a chance at a 3rd pool item. In total, you are guaranteed 4 items, with a chance at a 5th.