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
es
noun
The name of the Cyrillic script letter С / с.
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 es occurred in a corpus of books? (source: Google Ngram Viewer )