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
is
verb
Used in phrases with existential there when the semantic subject is a third-person plural.
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 is occurred in a corpus of books? (source: Google Ngram Viewer )