Number of diagonals = 18*(18-3)/2 = 135
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The formula for the number of diagonals (that is, lines joining non-adjacent vertices) belonging to any shape is n(n-3)/2. Therefore, an eighteen-sided polygon has 18 x (18 - 3)/2 = 135 diagonals.
All I can tell you is that a 16 sided shape is called a Hexadecagon
Tetrahaidecahedron
Octadecadron
If it is a 2 dimentional shape it is called an 'octadecagon'