Naive question from a non native speaker: What about "you" (second-person plural)? That's the one I learned in school. I (for the most part) transitioned to "y'all" because it is short and one can see the difference between plural and singular, but I wonder where and how often "you" is used in spoken language.
The technical answer from a semi-professional linguist, is that "you" is the official answer for the 2nd person plural in English. And it's what I see used most often. However, it can be confusing occasionally, so various regions have come up with ways to differentiate between 2nd person singular and plural, but the are considered colloquial and dialectical rather than official. And now back to your regularly scheduled humor stream.
So basically what Padre said. "You" is officially both the singular and plural of "You". However, regions have come up with ways to differentiate the 2. Whether that is combining 2 words (You and all) or simply adding an "se" at the end (Youse). You will generally only come across the slang version in speech rather than writing.