It's quite acceptable to call it a 99-agon but what's more important is knowing how to work out its properties.
I would call it a 99-gon. However, if you wish to annoy your audience (unless they are Greek) or be pretentious, you could call it an enneacontanonagon.
99-agon
ninetyanlamenteisfallernamnine is a 99 sided shape
The name of a 99 sided polygon is enneacontaenneagon.
The name for a 57 sided shape is derived by combining the name for a 50 sided shape with the name for a 7 sided shape. A 57 sided shape is called a pentacontaheptagon.
ennacontkaienneagon, or a ennacontenneagon
The name of a 51 sided polygon is Pentaconhenagon. The name of a 52 sided polygon is Pentacondigon. The name of a 53 sided polygon is Pentacontrigon. The name of a 54 sided polygon is Pentacontetragon. The name of a 55 sided polygon is Pentaconpentagon. The name of a 56 sided polygon is Pentaconheptagon. The name of a 57 sided polygon is Pentaconhexagon. The name of a 58 sided polygon is Pentaconoctagon. The name of a 59 sided polygon is Pentaconenneagon.
ninetyanlamenteisfallernamnine is a 99 sided shape
The name of a 99 sided polygon is enneacontaenneagon.
nonoetregon
The name for a 57 sided shape is derived by combining the name for a 50 sided shape with the name for a 7 sided shape. A 57 sided shape is called a pentacontaheptagon.
ennacontkaienneagon, or a ennacontenneagon
a nonadecanonagon
The name of a 51 sided polygon is Pentaconhenagon. The name of a 52 sided polygon is Pentacondigon. The name of a 53 sided polygon is Pentacontrigon. The name of a 54 sided polygon is Pentacontetragon. The name of a 55 sided polygon is Pentaconpentagon. The name of a 56 sided polygon is Pentaconheptagon. The name of a 57 sided polygon is Pentaconhexagon. The name of a 58 sided polygon is Pentaconoctagon. The name of a 59 sided polygon is Pentaconenneagon.
The name of a tweleve sided shape is a dodecagon.
It is a 99-sided 2-dimensional shape (polygon).
Decagon. A decagon is the name of a ten sided shape.
A 10-sided shape is a decagon.
icosikaidigon is a 22 sided shape