14.5.9 exercise 9: Quanto / quanta / quanti / quante
Quanto shouldn't be confused with quando. Quanto means how many / much, quando means when.
Di quanto ne hai bisogno?
How many do you mean?
When does he come?
Beside that there is a fundamental difference between the singular and the plural. With the singular we ask for the amount of something, the singular must be translated with how much (meal, water, milk). With the plural we ask for the number of elements of something and the English translation is how many (cars, pencils, chairs). In both cases quanto can be used as an interrogative pronoun (substituting a noun: Quanti vengono) and as an interrogative adjective (Quanti uomini vengono). In the case that it is used as a noun obviously everybody participating in the conversation has to know what the interrogative pronoun stands for.