Dear Rick: Your Arborvitae will fill in easily as a single row as you have them planted. The first season, the plants focus on rebuilding a root system and once the roots recover, much more needle growth will occur annually. Over planting and crowding them together will cause the body of the plant to thin out and that is the opposite of why you planted the screen in the first place. Be sure to water the planting during hot dry weather, especially while they are young.
Hope this is helpful, Richard