We don't have perceptions of countable or uncountable in our minds, we just know what sounds right according the the grammar. It's simply a question of whether a word in a particular context is singular or plural. 'Homework' is a singular noun, so native speakers instinctively know that 'How many homework have you got?' is wrong. We'd say 'How many assignments ?' 'How many exercises?' , because these are words in the plural. 'Homework' is a singular noun - there is no such word as 'homeworks' - so we instinctively know that 'How much homework have you got?' is correct. So, no concepts, no perceptions, no deep meanings - just the simple grammar of singular and plural.