It's about a girl named Nana in a setting where there are talented individuals. Nana is not what she appears. The light novel delves into her complex character as she tries to carry out her task among the talented students on the island. There are elements of suspense as the other characters gradually discover things about Nana and the true nature of their situation.
Nana is one of the main characters. She is the central figure with her hidden mission. There are also other talented students on the island like Kyouya, who plays an important role in the story as he starts to suspect Nana's true nature.
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.