Number agreement systems often present traces of older elements common to different languages of the same family; thus, their emergence is difficult to reconstruct. One possible origin of such systems is the grammaticalization of plural words into bound morphemes, which, as a result of a long process, develop into agreement markers and may become obligatory. Various investigations have provided evidence for this hypothesis. However, the complete process of change from a system with no number as grammatical category into a number agreement system in a single language has not been documented. This paper analyses documents covering different stages of the development of the Nheengatu language from Tupinambá in order to observe how the number agreement system emerged in modern Nheengatu. By doing so, this paper supports the idea that grammaticalization may have occurred rapidly in intense contact situations.