A 13 sided polygon is called a tridecagon or a triskaidecagon. The name comes from the Greek words for thirteen.
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A five sided polygon is called a pentagon because pentagon means five sided polygon I think.
A 12-sided polygon is called a dodecagon.
A 20-sided polygon is called an icosagon.
A million-sided polygon is called a megagon.
A 90-sided polygon is called an enneacontagon.