There is some difference of opinion regarding the use of apostrophes in the pluralization of references to letters as symbols. New Fowler's Modern English Usage
us
pron
Me.
Our.
Me and at least one other person; the objective case of we.
Me (in all contexts).
det
The speakers/writers, or the speaker/writer and at least one other person.
How often have the words There is some difference of opinion regarding the use of apostrophes in the pluralization of references to letters as symbols. New Fowler's Modern English Usage and us occurred in a corpus of books? (source: Google Ngram Viewer )