Individual letters that are used as letters should be italicized. When the letter is lowercased, an apostrophe s is used to make it plural. When the uppercase letter is used, an apostrophe is not usually needed.
Mind your p’s and q’s.
Put your X on this spot.
There are too many Xs on this page.
Scholastic grades are capped and set in roman type.
I got an A in English and a B in French.
Jan had straight As.