Just for the curious people, Nana's name at first is this kanji (名無) from the word 名無し (nanashi) = No name. Now her name is (七名 = Nana) which literally means "7 names", encompassing the 7 names written for her to choose, as Kazunari explained.
oooooooooooooooooooooooooooooooooooooooooohhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh mmmmmmmmmmmmmmmmmmmmmmyyyyyyyyyyyyyyyyyyyyyyyyy loRDDDDDDDDDDDDDDDDDDdd i cant believe how fluffy this is gonna be. theres so much backstory i cant wait to know!!!! love it.