It's quite acceptable to call it a 300-agon but what's more important is knowing how to work out its properties.
A 300 sided polygon is known as a trictogon
A 360 sided polygon is known as a Trictohexacontagon. Tricto=300 Hexaconta=60
The name of a 12 sided polygon is Dodecagon.
A 8 sided polygon is called a octagon.
name of 4000 sided polygon
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.
The name of a 99 sided polygon is enneacontaenneagon.