What is the plural form of the word "punctuation"?
It doesn't have a different form for the plural.
Like a collective noun, "punctuation" refers to all of the punctuation marks in a work.
You could say to someone, "You have a problem using correct punctuation," and this would be a general reference to a person's writing.
To be specific, you can say, "Three of the punctuation marks in your essay are incorrect," or "Please go through and correct the punctuation in your essay."
You can say "punctuation marks" for plural.