What is the plural for Audi?

Audis is the plural form
Audi's is the possessive form
Audis' is the plural possessive form

The plural of Nazi is Nazis.

The apostrophe is only used for possessive or to indicate missing letters, as in "it is" -> "it's", or "can not" -> "can't"

Wiktionary.org/ does not have one noun that I could see that had a plural created by adding "'s", with the exception of "diva's" which I think can be contested.

The same goes for abbreviations like CD and DVD -> CDs and DVDs.