The answer depends on whether you want to help your readers or you wish to be obfuscatory or appear pompous.
In the first case, the answer is 120-gon. Everyone will know exactly what you mean.
For the second, follow the attached link.
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A 2d nine sided shape is called a nonagon.
A 2D sphere is a circle.
A 2D shape with 9 sides is called a Nonagon.
2D shape
Hendecagon