What is a 99999999999999999 sided shape called?

Answer 1

Not all plane figures have an individual name that would be impossible to do, this is an example. It is simply a multi-sided polygon.
Some people refer to them as the number of sides adding the suffix -agon, so it can be called a
99999999999999999-agon

Answer 2

TIf you wish to communicate with others I would suggest you call it a 99999999999999999-gon. If, on the other hand, you want to confound your audience, then learn Greek prefices.

Answer 3

It is called Super Megagon.