Tanto How much quanto How many são used in questions sempre que queremos saber a quantity de algo. But… Você sabe qual usar?
‘How many ‘ is used apenas com o plural de coisas countable:
___Eg: How many apples did you eat?
Já ‘How much’ is used com coisas uncountable:
___Eg: How much sugar would you like in your coffee?
___Eg: How much milk did you drink?
Oh! ‘How much’ is used para ask the price, too:
___Eg: How much is the cereal?
Quer saber more sobre How much and How many? Access http://www.grammar.cl/english/how-much-how-many.htm para aperfeiçoar your grammar.