When rehydrating Kombu you can discard the water to reduce the iodine content. This is optional as some of the flavor and other minerals will be lost in the water.
Depending on the size of the rice balls you can make 4-6 onigiris. Watch the video to make them correctly.
Although they can be eaten at room temperature, onigiri should be formed with the rice still warm. It will be easier to form and they will keep compact and not fall apart.