I'll work a little more in on the subject above.
Multiples, they can be used to weave some continuity into a design. Clustered together or spread out, they have the power to make the word naturalistic come to mind.
One other thing to consider, while we are thinking about how plants tend to grow in colonies or just more frequently in ideal habitat, is the natural arrangement of plants, with respect to their environment and one another. Plants are always in competition with each other for resources, most of all a good spot in their desired light intensity, which is necessary for them make a living. A lot of tropical terrarium plants (especially terrestrials and vines) really don't need much light and so they are more difficult thwart. But epiphytes are usually trying to find a spot with exposure to brighter lighting or at least away from the competition for lighting found lower down.
One of the first things you might note, during a review of in situ epiphyte photographs, is that they are quite often growing at some kind of angle with respect to horizontal, and to each other. As was discussed above, epiphytes are often seen in groups of the same species, especially bromeliads. Accordingly, placing epiphytes at some kind of angle from the others can aid in developing a naturalistic appearance. Placing five different bromeliads in a line does not hit upon a natural theme necessarily, but it is not impossible to find something comparable in nature - but it is not as easy to relate to it. But again, some tree branches are nearly level.
Of course, not all tanks can accommodate large amounts of multiple plants, and that is ok. Sometimes it is better to use one individual of a plant, such as a bromeliad, since they can be found as singles in nature and it will benefit the other plants and the overall appearance of the vivarium to do so.
This concept is possible for terrestrials too, since they can also be found growing on nearly vertical surfaces, such as banks and steep hill sides.
See some examples of epiphytes growing in nature:
More to say about angles later.