"I need some water."
Translation:Potrzebuję trochę wody.
I'd say it can refer to countable objects: "niektórzy ludzie lubią owoce, inni lubią warzywa" (some people like fruits, others like vegetables). Also, I think that "niektóry" and other singular forms are rather a theory, they may exist in a table but I struggle to imagine them being used. "niektórzy" is, at least in my opinion, a plural pronoun.
"trochę" is "some" as in "a bit of", it mostly refers to the amount of something, usually uncountable I think, although it's surely not restricted to it. It can also mean "slightly/a bit", as in "Trochę tu gorąco" (It's a bit hot here).