kiom

One word covers both English questions, because Esperanto does not care whether the thing is countable: kiom da akvo and kiom da homoj are built the same way. The da is compulsory before the noun and is the bit learners drop. Kiom also works as a plain adverb of degree with no noun at all, as in kiom pli longe. It pairs naturally with tiom in a correlative sentence: kiom vi donas, tiom vi ricevas. There is no need for kiom multe, which is a translation from English rather than Esperanto.