Unlike the song, "one is the loneliest number" in the landscape it's more like a stand out number as one plant is considered an interest plant. Something to show off and be proud of like this Monkey Plant for all to see.
Threes a charm in the landscape and to the eye. What I like to do is try different configurations. These birds of paradise offer a pyramidal shape with the tallest in middle. Three planters offset from each other offers a great way to breakup monotony. When planting in threes you don't have to have three exact of anything. You can bookend a couple of plants then feature something different in the center.
Mass plantings can be considered where five or more plants come together. But, you know what I really like? It's when nature talks to me in odd numbers. Look at this cacti clump and count with me the number of plants that are mounding up (1, 2, 3, 4, 5, 6, 7, 8, 9). I don't know about you but I'm not going to argue with nature so it's odd numbers of plants for me.