Hi there,
I've just had a chance to look at some of the servo motors, and nice observation skills, I've never noticed that the instructions only show 12 holes before.
All of the white servo horns (the white discs) I've used have had 18 holes in them, and they've all worked fine. You need to use the smaller screws that come with the kit, and aim for the central hole of each set of 3. Looking at some of the pan/tilt kits I've constructed you also want to aim for the inner row when putting a scrrew into the set of 6. The fit of the screws into the holes will be tight but the idea is that the screws self tap and cut into the servo horn as they're screwed in. I recommend that you attach the servo horns to the black metal brackets before attaching the servo horns to the servo motors if possible so that you can use a bit more force to screw in the screws and not run the risk of damaging the servos.
Hope that helps, if anything was unclear, please let me know.
Regards
Alan