An eighteen sided shape is called a Octadecagon, or a Octakaidecagon.
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Provided we are talking about a 2D figure with straight sides, it is an octadecagon.
A one sided shape
Any of these shapes is practically circular. A 20-sided shape is an icosagon. a 50-sided shape is a pentacontagon (aka pentecontagon). A 100-sided shape is a hectogon (aka hecatontagon). A 1000-sided shape is a chiliagon.
a six sided shape is a hexagon
A twelve sided shape is a dodecagon.