I want to know which word I should use in the following sentence:
How many/much feces does a human produce in one year?
I found that both versions exist on the Internet.
Any help would be appreciated!
I would avoid this construction altogether and go for something like
How much fecal matter does a human produce in one year?
Indeed, many feces is incorrect, as feces are uncountable. Much feces is also quite incorrect (and sounds awkward, too), as feces is plural by default. Not all uncountable nouns go with much. For instance, scissors is a plural uncountable and both much scissors and many scissors are incorrect. Instead, one says many pairs of scissors to indicate the plural. Thus, you could also introduce a unit of measurement to quantify feces in your example:
How many kiograms/pounds of feces does a human produce in one year.
Much. Feces cannot be counted individually, so it is used with much. Many is only used with countable nouns.
"Much feces" is correct. Even though it ends in -s, feces is not plural and cannot be counted.