It's on the first screen when you visit your garage. Go there, change to a car that allows you to select a number, back out to the first garage screen again (where you can see your car and driver) and it's at the bottom of the list of options on the left.My numbers are #3 #7 #6 and #27.
OT: where I can change my number? I found it in GT5, but in GT6 I didn't find it... I am missing something?
I never use just one number. Its just the number of the car in my garage. So for example if I have 7 cars that can be numbered, they are numbered 1 through 7.