How to Make It
Preheat oven to 425°F. Arrange necks in a single layer on 2 rimmed baking sheets. Bake in preheated oven until deeply browned, 30 to 40 minutes, flipping halfway through cooking.
Pull duck meat from bones; reserve meat for another use. Transfer bones to a large stock pot; scrape any browned bits and juices from baking sheets into pot. Add 10 quarts cold water, and bring to a boil over high. Reduce heat to medium-low, and simmer until stock is dark and reduced to 4 1/2 quarts, about 4 hours, occasionally skimming impurities off of the top with a fine wire-mesh strainer.
Pour stock through a fine wire-mesh strainer into a bowl; discard solids. Let cool at room temperature 2 hours. Cover and chill 8 hours or up to 5 days. Skim off solidified fat on top of stock before using; discard fat.