Let's take a look at how plural letters can be written, with some examples from Harry Potter.
The main purpose of using correct grammar and punctuation is to make your meaning clear.
Hermione reminded them, "This essay is for Professor Snape, so make sure you've dotted all your i's and crossed all your t's."If you wrote "dotted all your is," readers would see the word is and be confused as to what you meant until they untangled the rest of the sentence. Distinguish the lowercase letter itself from the pluralizing "s" so that the reader will mentally pronounce the letter rather than making it part of the word—as "eyes" not "is."
Ron hissed across the library table, "How many i's are there in 'Wingardium?'"
This is a rare instance in which style guides will approve the use of an apostrophe for forming the plural, and you can see why you need to distinguish letters such as "i's" and "u's."
Most style books recommend using an apostrophe with lowercase letters, and the Chicago Manual of Style recommends the usage as illustrated in the examples above: italicize the letter and add the apostrophe, except in the case of common phrases such as dotting your i's and crossing your t's or minding your p's and q's, in which case the apostrophe alone is sufficient. New Hart's Rules (Oxford University's stylebook for British English) says that apostrophes aren't necessary if you are using italics for the letter.
Arithmantic runes are a far cry from the xs and ys of algebra.
When it comes to capital letters, the recommendations are also varied.
Once Hermione came to Hogwarts, earning As wasn't good enough; Acceptable wasn't anywhere near Outstanding.Again, there are some instances in which there could be confusion with no apostrophe.
Since magical education doesn't have any curriculum as basic as the three R's, it's a constant political battle within the school's Board of Governors to balance the class offerings.
"You should earn all Ms, Granger," Draco sneered. "'M' for Mudblood."The Chicago Manual of Style recommends no apostrophe with capital letters, unless there could be confusion. The Chicago editors usually recommend no apostrophe with letter grades, although you can see in the last example that "As" starting a sentence reads as the word "as." Other style books, such as the Associated Press Stylebook, recommend always adding an apostrophe with a capital letter for clarity and consistency. These examples should be written as:
"As are not sufficient to ensure entry to N.E.W.T.S. level Potions," Snape insisted.
"You should earn all M's, Granger," Draco sneered. "'M' for Mudblood."
"A's are not sufficient to ensure entry to N.E.W.T.S. level Potions," Snape insisted.
In the end, the answer is to make sure that your plurals of individual letters do not confuse the reader.
Lower case letter:
Always distinguish the letter from the "s" when lower case—with italics, an apostrophe, or both.
With a capital, definitely use an apostrophe when needed to avoid confusion. You may choose to use an apostrophe with a capital letter that doesn't form a word with "s" or not.
The main thing, with both lower case and capital letters, is to choose one style and apply it consistently.
Chicago Manual of Style
Associated Press Stylebook
New Hart's Rules