The 8 and 10 wraps refer to the amount of times the copper wire is wrapped around the cores in the individual coils.
8 Wrap Coils
The 8 wrap coil is used mostly for lining, it takes less voltage to push smaller coils. When using a single needle (3 or 5 outliner), the 8 wraps is a good way to go. With good machines you rarely need more than 8 wraps and you shouldn’t need more than 7 volts or so to hit hard.
10 Wrap Coils
When it comes to shading, or thicker outlines, you need to increase the voltage. A 10 wrap coil is great for this. Driving a 7 magnum into skin is no easy task, so naturally more power is required. A 10 wrap can handle more voltage, but 12 volts is the maximum amount you would want to put into your machine. You might break a spring if you run it too high too long.