How do I choose between 3-ply, 5-ply and 7-ply board?

Match the board to the protection you need. 3-ply single-wall suits short-haul light packaging and is the best value; 5-ply double-wall is the balanced all-rounder for appliances and electronics; 7-ply triple-wall carries up to 500 kg and is made for replacing wooden and steel crates around heavy equipment.