The first thing you need is determine which type of watercraft you have.  Is it a pontoon or V-hull boat?  If it is a V-hull then you just need one set of bunks set at the height that provides clearance of the bottom of your boat above the frame of the cradle of the lift.  If it is a pontoon with dual-toons you are going to need two bunk sets and three sets for tri-toon pontoon boats.