One of the main themes is friendship. The bond between the two Nanas is central. They support and rely on each other through different situations. Another theme is love and heartbreak. Nana Komatsu's numerous romantic entanglements and Nana Osaki's past love that still haunts her contribute to this. Also, the pursuit of dreams, especially in the context of the music and fashion industries, is a major theme. Both Nanas are striving towards their goals in these fields.
The full story of 'Nana' manga is centered around the two Nanas. Nana Osaki is determined to make it big in the music industry with her band, Black Stone. Nana Komatsu, on the other hand, is constantly getting into romantic messes. Their friendship is a key aspect. They support each other through various hardships like Nana Osaki dealing with her ex - lover's influence on her music career and Nana Komatsu facing heartbreaks. The story also delves into the lives of the people around them, showing different facets of modern - day Tokyo life.
To be honest, I don't have the exact count of how many Nana manga there are. It's best to refer to official manga sources or dedicated manga communities where you can get a clear and up-to-date number.