While that's happening, pour the 12 gallons of water into a 20-gallon pot. Add the remaining salt, the seasoning, cayenne pepper, black peppercorns, bay leaves, and lemons. Boil for 15 minutes. In a basket insert, place half the garlic, half the onions, and half the potatoes. Place the basket in the water and cook for 4 to 5 minutes. Add half the corn and return the water to a boil.
Drain the crawfish from their purging water, add half of them to the basket, and bring to a rolling boil. Turn off the heat and let soak for 10 minutes. Pull out the basket, drain, and dump the basket's contents onto a newspaper-lined table. Repeat with the remaining ingredients.
Serve crawfish, corn, onions, and garlic to each person.
Note: You certainly can boil a small amount of crawfish in a smaller vessel, do it indoors, and serve it to a smaller gathering, but in New Orleans, this is for an outdoor bash.