Combine chile flakes, dried thyme, parsley, peppercorns, bay leaves, lemons, and 16 cups water in an 8-qt. saucepan and bring to a boil. Add crabs and boil until cooked, about 15 minutes. Using tongs, transfer crabs to an ice bath until chilled. Transfer cooking liquid, herbs, and lemons to a bowl and set aside. Clean crabs, reserving crabmeat for soup. Discard top shell