Depends on how big (long) the island counter is; it could require more than one. Sometimes, it's nice to put the outlet IN the countertop. This would allow you to plug in things and not have the cord hanging over the edge. Wait for the pros for a real answer. You really need to ask the inspector what he wants to pass it. What is required would somewhat depend on which code applies where you live...they are not the same everwhere. For yours, I'd consider one on each end. I think they like one every 4'., but I suppose one in the middle would keep it below that distance.