The Russian name for a set of wooden Russian Dolls is matryoshkas, and there can be any number in a set, and they can be any size too.
In 1970, a matryoshka was made to celebrate the Soviet Communist leader Lenin's birthday, and it was one metre tall, and contained 72 dolls in all.
O'Farrell uses the metaphor of the Russian Doll to convey the various identities that Lexie will have as she develops. It is a popular metaphor, that some self-development organisations have picked up on as a tool for teaching. It has also been found useful by a blog article on the New Scientist, as a way to convey the five stages through which the human brain develops over a person's lifespan.