ni is indeed typically plural, although it can be used in the singular (which we don't recommend - long story).
The most common way of expressing this in English is just "you", but since "you all" or "y'all" is very common in large parts of America, we try to accept those two. However, they're missing in a lot of places, so when you encounter that they don't work, feel free to report them. I've added them to this sentence now. :)