While category of number exists in majority of languages of the Earth, it is not native to Black Speech. It's believed by some linguists that it was absent in Black Speech. This opinion is based upon using the word “Nazgûl” as both singular and plural noun in “The Lord of the Rings” trilogy. The other reason is the popular belief that Sauron suppressed individuality and forbade to use the word “I” or even any other singular pronoun. This resembles dystopian novel "We" (1921) by Yevgeny Zamyatin. However from LOTR it's clear that Sauron failed in organizing orcs who regularly quarrel with each other and put their personal wishes above the order (i.e. scene of Merry and Pippin escaping orcish capture).