Several researchers have suggested that languages vary as to whether number marking on noun phrases is obligatory or optional (Arka & Dalrymple 2017; Biswas 2013; Bylinina & Podobryaev 2020; Corbett 2000; Kim 2005). We develop an implementation of this idea within a theory that assumes that the plural is a semantically vacuous, unmarked number (Sauerland, Anderssen & Yatsushiro 2005 and others). We then explore the semantic consequences of this proposal. One striking prediction of the proposal is that plural nouns should be ungrammatical in antitone environments in languages where number marking is optional, i.e. plurals should be positive polarity items in optional number languages. This prediction arises because the plural form is equivalent to the number neutral bare form in optional number languages. The Efficiency requirement of Meyer (2013) therefore predicts that plurals require local exhaustification in optional number languages since otherwise the bare form is more efficient. Because bare forms are impossible in obligatory number languages, plurals are not predicted to be positive polarity items in such languages. We conclude with an outlook on how to empirically verify the predictions of the theory we presented.