Does the word mucus have a plural? If so, how would it be spelled?

muci

Number Singular Plural
nominative mūcus mūcī

http://en.wiktionary.org/wiki/mucus

"Mucus" is a collective noun, like "blood" or "lymph" or "phlegm", and doesn't have a plural.
http://oxforddictionaries.com/definition/mucus?view=uk

This site refers to nouns of this type as a "mass noun"; another site refers to them as "uncountable":
http://www.macmillandictionary.com/dictionary/american/mucus

Let me elaborate a little: the English word "mucus" doesn't have a plural form, but the Latin word "mucus", from which the English word is taken, would have the plural form "muci" (though I can't imagine any plural usage in Latin, either).

Source: the above sites and 11 years of studying Latin.
The Wiktionary page cited earlier refers specifically to the Latin "mucus", which is why there's an apparent contradiction. That page also lists the English "mucus" as "uncountable".