Preheat oven to 425°.
Crush 8 garlic cloves with the flat side of a chef's knife and remove peel. Cut ½ stick butter into small pieces.
Empty 28 oz. can of tomatoes into a 13x9" baking dish and crush lightly with your hands.
Scatter garlic, butter pieces, 2 anchovies, and ½ tsp. red pepper flakes over tomatoes; season with salt and black pepper.
Roast tomatoes, tossing halfway through, until garlic is very soft and mixture is thick and jammy, 35–40 minutes.
While tomatoes are roasting, bring a large pot of water (4-6 qt.) to a boil over high heat and add a small fistful of salt.
Remove pan from oven and let sauce cool slightly. Using a potato masher or fork, break up garlic and tomatoes until mixture looks like coarse applesauce.
Cook 12 oz. bucatini or spaghetti in pot of boiling water, stirring occasionally with tongs, according to package instructions for al dente.
Toward end of cook time, use a heatproof measuring cup to scoop out and reserve ½ cup pasta cooking liquid. Drain pasta in a colander; reserve pot.
Return pasta to pot and add tomato sauce and pasta cooking liquid. Cook over medium-high heat, tossing with tongs, until sauce coats pasta, about 3 minutes.
Divide pasta among plates. Serve topped with Parmesan and more red pepper flakes.