I'm also a native speaker and the sentence sounds fine to me. The grammar is perfectly correct. Now, present tense can also be translated as you probably know as a progressive: "How many pieces of cake are you eating?" but it all depends on context and I'm sure there are a number of scenarios that could warrant questions phrased either way.
Agree! It doesn't seem very natural as a 'stand alone' question...but if I said, "I'm not invited to the birthday because I eat too much cake," and you replied, "How many pieces of cake do you eat?" ...probably preceded by 'Really!!' ....you can see how it works in that context. Italian structure is often quite different to English and therefore you'll find quite a few odd translations...where the grammar is sort of ok, but it just doesn't seem 'natural' in English. Just go with it....and good luck with your language learning.
Of course it's correct. It's just that it's a question that's unlikely to be asked, because as you said it sounds odd. Clearly it's syntactically correct. You'd be more like to say "How many pieces of cake would you like?" but that goes beyond the simple grammatical tense being used here, since this particular unit is not about the conditional. Just go with it.
No, the plural of the word piece / pezzo (masc) is pezzi, so how many pieces is 'quanti pezzi'. It has nothing to do with 'torta'. Furthermore, in the question 'quanto costa', the word 'costa' is not a noun it's the verb "costare" (3rd. sing), so that phrase is asking 'how much does it cost'.
Ah, sorry about that! I see what you're saying. It makes sense. I also had NO idea about the verb "costare" because I learned it solely as a phrase in my Italian class and I never really thought about it much, but I get it because "costa" on its own would be "(it) costs", right? I was curious, I had a feeling I had some misconceptions lol. Thank you, kind fellow! A lingot for you :D!
Makes no sense at all in english in present tense. So does it mean how many do you intend to eat or how many have you already eaten or how many are you eating now. Ok I can think of a situation but it sounds like future tense - Q. how many pieces of cake do you eat when you are very hungry?