What is the rule here to use partitive for kuppi or not? Is it yksi being only one, thus countable? If so, then why is kahvia partitive (as in incomplete or non-countable)? Isn't one cup of coffee completely countable?
All numerals except "yksi" trigger partitive case in singular form. Nominative case is used with "yksi", which is why "kuppi" is in nominative case. Apart from "yksi" and "muutama", anything that expresses an amount or number of something triggers partitive case, which is why "kuppi" gives partitive case to "kahvi". It expresses the amount of it. The fact that it has "yksi" behind it doesn't change that.
Thank you. I needed that. My intuition for partitive is still a bit shaky, so thank you.