Personally… I do two+. It depends what kind of suit you will use. Trilam or neoprene. But, always bring double sets. A drysuit is only as dry as the next potential leak. They get fewer with your own suit, but it is nice to have new dry undergarments in case of a leak on a class.Is a wool base layer enough under the thermal layer? (sorry if asking basic questions here)
Thermos filled with hot water is noted
Water is 3-5c at the moment.
Bring a wool beanie. Wool neckliner and something insulating to stand/change on. (Sitteunderlag feks)