How to say, "Many are not one?" Is it: pluribus non paribus unum?
The sentence has more than one possible meaning in English that might slightly alter the way you may want to translate it into Latin.
Suppose that you want to put emphasis in something like the fact that a group of individuals usually lacks the complete unity of a single organism/person/entity. This seems an obvious meaning for the sentence, especially judging from the philosophy-terms flag.
A possible (mostly word-by-word) translation that fits this sense is:
Plures non sunt unum.
- Many in this context may be translated as plures, multi or to a lesser extent, plurimi. These are plurals. Plures may be both masculine or feminine, while the others are masculine-only.
- Note that pluribus is a derivative of plures that does not fit here. In Latin, the ending of nouns gives grammatical context: while plures may be used as the subject of a sentence or as complement of copulative sentences (e.g. after the verb to be), pluribus has to be linked to a preposition (as in e pluribus unum) or be the indirect object of a verb.
- The verb to be can be omitted in Latin, it just sounds better to me not to in this case. (But I can't see an error in writing plures non unum.)
- You could also alter the sentence order: plures non unum sunt.
- I translated one as unum, which is neuter in gender. I preferred this assuming that one refers to the philosophical concept of a unity, as opposed to one single person. Unlike unum, the neuter version of plures (plura) or multi (multa) would make it sound as many things instead of many people. In Latin you would only use pl. feminine (multae) if the many are all women.
- I chose not to add pares or aequales (meaning equal to, e.g. plures non sunt pares/aequales uno, plures non pares uno), since that emphasis was not in the original English sentence.