A hexahedron is the name for a 6 sided 3D shape.
the name of a 20 sided 3d shape is called a icosahedron
There is not a definite name for a 3D figure with 18 sides.It is possible that it is something else - but the information to test this is not provided.
Octadecadron
HEXADECAHEDRON is the correct name for a 16 sided 3D object
A 50-sided 3D shape is called a pentacontagon. It is a polygon with 50 sides and 50 angles. In three dimensions, it would be a polyhedron with 50 faces, 50 vertices, and 100 edges.
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Tetrahedron
Dodecahedron
dodecahedron
Octahedron
A 12 sided 3D shape is a dodecahedron.