I'm not that expert in historical things, but I get through the reading from other sources...
The name of N9 (acronym for Negeri Sembilan)is originated from the16th century. It is including the associated with the places which was opened by the Minangkabau people of the Malay Peninsula which is now part of Johor, Naning is now part of Melaka, Rembau, Sungai Sawan, Jelebu Johol and now is the integral part of Negeri Sembilan.
For further reading in the history of N9, u may read here
There are eight districts in N9. (Seremban, Rasah, Telok Kemang, Jempol, Jelebu, Rembau, Tampin, Kuala Pilah)
In clarifying how N9 developed, these sentences may help u to imagine it....
Taratak mula dibuek
sudah taratak menjadi kampuang
sudah kampuang menjadi dusun
sudah dusun menjadi kotosudah koto menjadi nagari