Clothes is an example of a plural-only noun. There are a small minority of nouns in English which, when functioning as the heads of Noun Phrases, only occur in the plural. A few more examples are:
- police, people, scissors, trousers, arms, groceries.
Notice that not all of these are marked with the regular plural ending 's', for example: police. Police has no plural suffix at all - either in the writing or in the sound. As noted by Snailboat in a helpful comment below, those nouns that are marked for plurality can often occur without their plural 's' suffix as modifiers in other Phrases:
- a trouser press
- She's a proud upstanding trouser-wearing feminist
This phenomenon aside, these nouns normally only occur in the plural, they take plural verb forms, and importantly, only take quantifiers that are compatible with plural countable nouns. They don't permit quantifiers that are usually used with non-countable nouns. So for example they can often occur with a few and many and with numerals greater than 'one', but never with with much:
- many people, many trousers, many clothes
- few poeple, few trousers, few clothes
- five thousand police, ten trousers, two people,
- not much people*, not much trousers*, not much clothes*, (wrong)
Clothes is unusual because it has a non-countable cousin clothing:
many clothing* (wrong)
few clothing* (wrong)
two clothing* (wrong)
not much clothing
Clothing is used like other mass nouns. It is very useful for when we want to talk about just one garment, because we can use it with other nouns like item and piece in constructions such as:
a piece of clothing
one item of clothing
We can't do this with the noun clothes:
- one piece of clothes*
- two items of clothes*
So to answer the Original Poster's first concern, we have a choice between:
- I didn't realize how much clothing you had on the floor.
- I didn't realize how many clothes you had on the floor.
The latter sounds more natural - though it's still a bit clunky.