Litter is an uncountable noun, so it is always singular:
There's too much litter.
Look at all this litter. Someone should pick it up.
Does your city have a lot of litter?
When you use litter in a different meaning, such as a litter of kittens (when a cat has babies).
My cat is five years old and she has had three litters. (litters of kittens are countable)
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