How large a cast is needed for a production of Hamlet? How many actors (including extras) are necessary to do dramatic justice to Shakespeare's magnum opus? Yet, some of the most intensely satisfying theatrical endeavors (and I am thinking of some of the classic Twilight Zone scripts, among other examples) require no more than two persons on the stage.
In just such a way, I feel no artistic limitation in the least, when writing a piece of music for two performers. Let alone for five. In fact, if a composer is capable of making a compelling 20-minute piece for two players, an ensemble of five is a spectacle.