how much brine should i make for a 16-18 lb. turkey?

Yes, sdebrango is correct. Use a large enough container - like a 6-gallon container, and be sure that it will be cold enough wherever you place it. I'm guessing that neither your fridge nor mine can hold one, so be sure that your garage or back porch is cold enough that everything will brine at less than 40 degrees.

I think you will need between 2-3 gallons. The turkey has to be completely covered in the liquid.