Basically you want one at any low point where water could gather. For wings you want a drain hole in every rib bay at the inboard aft corner. I put two on each elevator, rudder and horizontal stabilizers. I can't remember how many are on the fuselage bottom, but you want one as far back as possible near the tailwheel mount area.

Like DenaliFox said, seaplane grommets create suction and will draw out the moisture and humidity. That's why I like them and recommend their use over the flat washers.