I'm speaking of how a tune is put together, which instruments are assigned to what note(s), and the overall feel and flow of the tune. And this goes for any genre. It doesn't matter if one instrument is used or one hundred. It's how you want your listener to hear the tune you are presenting.
I've had the opportunity to play some really great (and some not-so-great) arrangements. Arranging as a musical art form seems to be dwindling, with only a handful of current greats out there (James Swearingen and Joel McNeely come to mind).
If you plan on pursuing arranging, I urge you to listen and study up. Knowing the strengths and limitations of each instrument is important in making the song sound effortless. Your listeners (and performers) will love you for it !