hectagon is the answer
Chiliagon has 1000 sides. (In geometry, a chiliagon is a polygon with 1000 sides. The measure of each angle in a regular chiliagon is 179.64°.)
A polygon with 1,000 sides is known as a chiliagon. A hectogon is a polygon that has 100 sides, and a polygon with 8 sides is called an octagon.
A 100-sided polygon is called a Hectagon.A 1000-sided polygon is called a Chiliagon.
Chiliagon
Chiliagon
A polygon with 1000 sides is still a polygon but even a polygon of 100 sides gives characteristics of a circle. When demonstrating circles a polygon of 100 sides helps show the circle characteristics.
A one thousand sided regular polygon is called a chiliagon.chiliagon (a polygon with 1000 sides)
A 1000 sided polygon is called a chiliagon.
It's perfectly acceptable to call it a 1000-agon
I stand corrected the largest sided polygon is a myriagon. Source the MIT. 1000 sided
On etymological grounds I would be tempted to call a 1000 sided polygon a myriagon. I will also point out that for most purposes we would call it a circle, since with that many sides it very closely approaches circularity.