Is "How many more?" an acceptable translation? I've always thought of כמה as being more related to count nouns than mass nouns.
Please, what is the difference between "עדיין" and "עוד"? We were taught both for the english "still". Any difference between both?
For 'still' they're pretty much interchangeable. But עוד also means 'more', so in this sentence only עוד works.
what is the difference between עוד and יותר and would כמה יותר be acceptable
I think כמה יותר is sometimes acceptable. For example if someone tells you - "שים יותר מים" (put more water), you can reply "כמה יותר" (how much more).
wait but if יותר is used as "more than", shouldn't it instead be "שים עוד מים"?
Yes. שים יותר מים is actually saying "put more water (than there currently is)"
Why is "how much still" wrong? (E.g. "how much of this do I still have to endure?")
If it is just a question of English, I beg to differ. In my (anglophone) part of the world, "How much still?" would be quite acceptable. "How much more?" isn't a complete sentence, either, but people do use interrogative phrases like those as stand-alone utterances.