More on "Kentucky Fried Chicken", and maybe this will help overall.
This reminds me of the "information(s)" question that comes up often. There are things that one counts in English, and things which one doesn't count. Individual birds (chickens) are counted, but the meat (chicken) is not.
So, I can have "chicken" for dinner, regardless of whether I have half a chicken, a whole chicken, or two chickens (if I'm really hungry). I can go to the supermarket and buy "chicken", and it's not clear whether I bought four breasts, ten legs, 20 wings, or two entire birds. I could say we're having "chicken" for dinner, and you wouldn't really know what I mean, other than it's not beef or pork. I might also say we're having chickens, which would probably mean two or more birds cooked whole (perhaps cut into individual portions when served).
So, in a broad sense, "chicken" might be referring to more than one bird, but if you're actually counting, then you have to say "chickens".
Note that this is not the case with animals like moose or deer. In these cases, the singular and plural are identical, and you have to hope there's a verb or adjective to figure it out. Sometimes you can't.
"Fish" is even more complicated, but since no one asked, there's no reason to get into it.
Hope this helps.