South-Bauchi West (SBW) languages build a dialect continuum spoken in Northern Nigeria that has been classified as West-Chadic B. Their internal classification reveals a split between two subgroups: the northern subgroup (Geji, Polci) and the southern subgroup (Zeem, Dass, Saya). This genetic split is completed by a grammatical heterogeneity that surfaces in the morphological complexity of the Saya cluster, a subset of the Southern sub-group. The aim of this paper is twofold: (i) see if these differences are corroborated by the study of number; (ii) shed new lights on the genesis of SBW. To that effect, the first section presents an overview of SBW grammatical structure and genetic classification. The following two sections study number in Noun Phrases (noun plurals, modifiers), and in Verb Phrases (imperatives, pluractionals, and plural suffixes). The last section examines the relationship in SBW between number and related categories such as honorifics, associatives and singulatives. The conclusion introduces some nuances in this vision of a division between the northern and southern sub-groups, with the Dass cluster (e.g. Zoɗi) behaving like the Northern languages in that they share the same absence of nominal and adjectival plurals and pluractional derivation. The presence of those plurals in the other members of the southern sub-group (i.e. the Zeem and Saya clusters) seems to be an innovation departing from a situation where number is expressed only by the personal pronouns, the modifiers within the nominal system, and the imperative. Pluractionals, although they are widespread in many Chadic languages, seem to be an innovation of Zeem and Saya as well. These innovative features may have developed through the influence of plateau languages. The innovation has gone one step further in the Zaar language with the optional marking of number on noun modifiers. Finally, a marginal case of subject-verb plural agreement on the right periphery has appeared in four languages (Zaranda, Geji, Pelu and Diir), first in the 2nd plural, and then has spread to the 3rd plural in one of the languages (Pelu).