The formal name is dihectagon. Most people, including mathematicians, will use 200-gon since that effectively communicates the size of the polygon. But, if you want to appear pretentious or be obfuscatory, by all means use the Greek prefices.
A dihectagon
The name of a 200 sided polygon is dihectagon.
The name of a polygon with 15 sides is a pentadecagon
It's perfectly acceptable to call it a 200-agon but what's more important is knowing how to work out its properties.
There is no formula for how many sides there are in a polygon; the name of the polygon shows how many sides it has.
A pentagon is the polygon with 5 sides.
A polygon with seven sides is a heptagon.
There is no name because such a polygon does not exist. A polygon must have at least three sides.
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 6 sides is called a hexagon.
A polygon with ten sides is a decagon.