I was going to report on the wrong pronunciation of ch used in Latin for the Greek χ in borrowed words, but it emerges that in the ancient period Greeks pronounced it as an aspirated k - just as it was written in Latin c + h, not as the modern fricative [x]. So the audio is correct.
Quanti is the singular genitive of "quantus", how much, in which quantity. How large, how big...
You see it in quantity.
Can't be declinated, it's a non variable (as adverbs), indeclinable. As "quot" is an interrogative adverb.
How many. Asks for a number.
Here, in the exercise, it's an interrogative word, asking for a number, so it's "quot".
I believe that the Latin for milk is the neuter noun lac (genitive, lactis), so "How much milk?" would be "Quantum lac?". For a masculine noun, e.g. caseus (genitive casei), "How much cheese?" would be "Quantus caseus?". For a feminine noun, e.g. aqua (genitive aquae), "How much water?" would be "Quanta aqua?".