The following was taken from the article Layer it on: The art of layering necklaces by NWSource, Seattle, WA
The number of necklaces depends on what you choose. Two necklaces that complement each other in texture and proportion can be the perfect addition to a "busier" outfit or neckline. With three necklaces, you can get more creative. Just make sure you choose pieces carefully for balance. Beyond three, it gets a little trickier, and in some cases, tackier. If you're after this look, consider buying one of the many multi-necklace jumbled affairs available.
While the jewelry lovers I talked to emphasized that there are no specific layering "rules," they generally agreed that:
1. The chains should be of varying lengths and staggered.
2. Adjustable clasps and hooks are a plus when layering. That way you can adjust to the appropriate length.
3. Mixing metals such as gold and silver in varying shades creates diversity.
4. Mixing media like pearls, gems, and costume beads and baubles adds texture.
5. Before buying, dig through your own drawers for forgotten or unusual pieces. You may not have to spend a dime!
To create a put-together look, use a combination of elements that brings the pieces together. Two of the necklaces should share something in common, either a color, metal, shape, gemstone or texture. The third necklace should share something in common with one or both of the other two.