Number of Strands

The more individual wire strands in a wire bundle, the more flexible, kink-resistant, break-resistant, and stronger the wire is. But more strands cost more.

The lowest number of strands is 7 with one in the middle, 6 surrounding it.
The next level up is 19, which is another layer of 12 strands on top of the 7. After that the number varies, but 37 and 49 are common, then in the 70 to 100 range (the number is no longer exact). Even larger numbers than that are typically found only in very large wires.

For application where the wire moves, 19 is the lowest that should be used (7 should only be used in applications where the wire is placed and then doesn't move), and 49 is much better. For applications with constant repeated movement, such as assembly robots, and headphone wires, 70 to 100 is mandatory.