What does the turnout number (I.e. #6 turnout, #8 turnout) stand for?

The turnout number is just a simple ratio that describes the angle of the turnout.

For example, a number #4 turnout has a ratio of 4 to 1, while a #6 turnout has a ratio of 6 to 1. The smaller the ratio, the shorter the turnout, and the tighter the curve will be in the diverging route.