SG-90 has 21 gear teeth on the head.
SG-90's included servo horns utilize the gear teeth to stick on the gear head, so you don't need screws to attach it, but if you want to go further and design your own servo horns, you're going to need this.
The attached .f3d and .stl files are both optimized for the 3D printer I used (Afinia H-480). So instead of just printing the horn, you'll probably want to download the .f3d file, edit the isosceles triangle and optimize the tooth size according to your 3D printer and plastic material.
The design also doesn't include the holes you probably want to add, to have the horn do anything.
Each teeth consists of a isosceles triangle. You can edit the constraint to adjust the size of the triangle.