"How many pieces of cake do you eat?"
Translation:Quanti pezzi di torta mangi?
I'd put tu at the beginning, "Tu quanti pezzi di torta mangi?"; it's not rare to see it at the end, but more as an afterthought, "Quanti pezzi di torta mangi, tu?". I don't think it should be marked wrong though, duolingo is a little too strict with omitting the subject sometimes.
Personal pronouns are usually omitted in Italian, but it is not wrong to include them. However, if used, the personal pronoun usually would be before the verb, i.e. "Quanti pezzi di torta tu mangi?" or "Tu quanti pezzi di torta mangi?" .
Regarding number, both singular and plural are accepted. Just try "Quanti pezzi di torta mangi?" and it will be accepted.
It is also correct. You could report it.
Anyway, whenever using Duolingo, remember that each correct answer of each exercise is manually inserted and therefore the most literal translations are more likely to be accepted, unless they sound very odd. Changing word order without a good reason (eg emphasis) and using synonyms are prone to rejection.