The paper discusses the limitations on the realization of the number category of nouns in the structure of Russian idioms. The main source of the material was the vocabulary of the “Idiom Dictionary of the Russian Academy of Sciences” (2015). The relevant limitations are shown to be determined by general grammatical factors, factors of semantics (meaning) of idioms, as well as features of pragmatics and inner form. An interesting case of constraints on the formation on the plural can be considered the characteristic of the referent of a given idiom: the uniqueness of a person (luchshiy drug sovetskikh fizkul’turnikov), a political subject (bol’shaya semerka), an event (velikiy oktyabr’), the uniqueness of the place (lobnoe mesto), and the time period (kamennyy vek). The constraint on the singular is largely determined by grammatical factors (imeniny serdtsa). The features of the idiom structure can also influence the implementation of the number category: rhyme, rhythm, and unique components (e.g., outdated and constructed words). Grammatical constraints turn out to be more regular. Some semantic and pragmatic constraints can also be predicted. However, a significant part of the semantic restrictions and constraints associated with the model of the inner form are selective. In a systematic analysis of idioms, regular factors are described by general rules, and unique ones are specified for each idiom. The study of the number category in idioms is part of an important program to study the implementation of grammatical categories in phraseology.