It's quite acceptable to call it a 40-agon but what's more important is knowing how to work out its properties
I find the name "30-sided polygon", "40-sided polygon", etc. just fine.
name of 4000 sided polygon
The name of a 200 sided polygon is dihectagon.
The name of a 600 sided polygon is Hexcosiagon.
there is no such thing as two-sided polygon.
The name of a 40 sided polygon is Tetracontagon.
I find the name "30-sided polygon", "40-sided polygon", etc. just fine.
A 40-sided polygon has 40 sides. Count again, to be sure.
The name of a 12 sided polygon is Dodecagon.
name of 4000 sided polygon
A 8 sided polygon is called a octagon.
The name of a 9 sided polygon is Nonagon.
The name of a 19 sided polygon is enneadecagon.
The name of a 17 sided polygon is Heptadecagon.
The name of a 8 sided polygon is Octagon.
The name of a 10 sided polygon is Decagon.
The name of a 14 sided polygon is Tetradecagon.