icosihenagon
There is no formula for how many sides there are in a polygon; the name of the polygon shows how many sides it has.
There is not a name for such a polygon. A polygon with one million sides is called a megagon though..
the name of the polygon with 101 sides is hecatontakaihenagon
A polygon with 4 sides is a quadrilateral.
icosikaihenagon
It will have 21 sides or a 21-gon
An icosikaihena is a polygon with 21 sides
A polygon with 21 sides is called an Icosagon.
icosihenagon
The name of a polygon with 15 sides is a pentadecagon
There is no formula for how many sides there are in a polygon; the name of the polygon shows how many sides it has.
There is no name because such a polygon does not exist. A polygon must have at least three sides.
A pentagon is the polygon with 5 sides.
A polygon with seven sides is a heptagon.
There is not a name for such a polygon. A polygon with one million sides is called a megagon though..
Though many polygons have a specific name not all do. It is accepted practice to use the number of sides as the name if one is not assigned (or known). A 21 sided polygon does not have a specific name so could quite reasonably be call a 21-ogon.