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OK, so the term 'sound design' is thrown around a lot, and many people will refer to a number of sound practices as sound design. I look at it in a very broad way. For me, sound design is musical, and that is largely because I take all sound to be musical. The way I see it, Foley work is sound design, just as composing a piece a music is sound design. You are creating a sound 'world', designing it. Sound effects in a video game or a movie are not created in a vacuum; if you have a musical track playing in the background, you're going to design the sounds in contemplation of the song, otherwise you're just throwing globs of paint at a canvas and hoping it looks good.

Secondly, to answer the question: layer as much as you think is necessary. If it sounds good, then layering might just make what you had and liked in the first place unintelligible. Don't just layer on top of a sound because you figure other people would, do it because you think the sound would be better because it. In any case, it never hurts to experiment!