Meanwhile, cook all of the shells to al dente (two minutes less than the shortest cooking time listed on the box). Use a slotted spoon to remove them from the cooking water. Set cooked shells in a big bowl and toss with a drizzle of olive oil. Only about 28 shells will fit in your baking dish, but many will break or tear, so it’s good to have extra.
Spoon 2 1/2 cups marinara into the bottom of the baking dish. Stuff each shell with about 2 tablespoons ricotta mixture and nestle it into the sauce. Stuff about 28 shells, tucking them into the baking dish. Top with grated mozzarella.
Cover baking dish with foil and cook 20 minutes. Uncover dish and cook 5 minutes more or until the cheese is melted and the marinara is bubbling. Top with fresh herbs, if using, and serve.
Leftover marinara can be saved for topping leftovers or repurposed.