Responding in English, it’s looking for the standard English word “you”, which is used for both singular and plural — much as the standard English word “read” is used for both simple present and simple past. You don’t need to enter “you all” for plural “you”, or “thee” for singular “you”, to show that you can distinguish between them.
Since I can distinguish between singular “you” and plural “you” in my mind, I don’t find it helpful to type “thee” or “y’all” to distinguish them in a written form. Doing so wouldn’t further cement knowledge of the difference between singular and plural that I already understand.
You’d asked me “But don’t you think …”, which I’d understood as inquiring about my personal view. I find it hard to believe that the ability to understand the difference between singular and plural is unique to my head.
As to the magical number five, my guess is that Duolingo’s designers decided that limiting the number of replies in a discussion would be a lesser evil than the possibility of the presence of a horizontal scroll bar.