How do I know what is the right amount of edging, statuary, etc. to use in my yard without it appearing like “too much”?

You want it to be tasteful and sophisticated, not cluttered — yet your personal style should shine through. What you do should enhance, not detract from, the natural beauty of your yard. Here are a few simple guidelines to follow:
a) Decor that is scattered throughout the yard will have less impact than if you create groupings.
b) Stick with a theme if you want to make a statement; a stone-like edging will be more natural and rustic than a wrought-iron style edging for example.
c) Intersperse outdoor decor with live plants, flowers, and shrubs to keep the overall look lush and natural.