Pluralisation does not require an apostrophe. Thus 777s.
The possessive, however, does. Thus a 777's AVOD.

Abbreviations may. Thus 'phones. Which I commend other posters for using.

NB: not poster's, because that is singular possessive, not plural. Or posters', unless used in the possessive for more than one of you.

Posters = more than one poster.
Poster's = a poster's possessive post.
Posters' = more than one poster's possessive post.

It is not that difficult.

Next week we shall discuss gerunds. Please Google that for your homework