Yes—one light can produce very good portraits. A strong starting setup is to place your key light off-camera at about 45° to the subject, then use a reflector on the opposite side for fill. This gives a natural portrait look, though you may not get a distinct hair light or separate background light.

For more dramatic results, move the light farther to the side (toward split lighting, near 90°) and use the reflector to control how deep the shadows stay.

A very useful one-light trick is to work in a corner with white walls. Put the subject near the corner and place the light at roughly 45° to it. One wall becomes part of the background, while the other wall reflects light back onto the subject as soft fill. Because reflected light is weaker, it helps create a pleasing light ratio. Small changes in the subject’s position or angle will change the balance of main light and fill.

You can also feather a softbox or umbrella so the edge of the beam lights the subject while more light falls on the background for a moodier look.