I typically start by choosing a bracelet(s) that i want the dominant color of the stack to be.
Then select other bracelets of the same stone (or of a stone with similar coloring) that will showcase the dominant color of the stack.
Choose a color to accent the stack. MIxing/repeating the same two materials (such as Rose Quartz and Howlite) is an excellent way to create harmony.
You can achieve a co-ordinated look by selecting the same stone with a different texture, bead size or finish (such as faceted/smooth, 8mm/10mm, polished/matte).
You can create extra interest by mixing accent materials but be careful not to mix too many different accent types (combining brcelets with gold and silver spacer beads, for example, achieves a delightful syle)
When it comes to how many, personally for me, 3-5 bracelets seems to be the magic number to wear when i'm layering. Anything more than that feels a little too heavy. But remember to play around and find what's comfortable for you.