Number of Rostov children

No matter how many times I have read War and Peace (though I have never read it in an academic setting), I am always bothered by the apparent dissappearance of Rostov children.  When we first meet Countess Rostova, (Book I, Chapter 7), we are told, "The Countess was a woman...who looked worn out from childbearing; she had given birth to twelve." As we continue to read the novel, we clearly meet and delve into the characters of Vera, Nicolai, Natalya, and Petya, and no others.  Even when Petya's death is learned by the Count and Countess, it seems clear that he was the youngest; there are no others.  Was this an oversight by Tostoy? Is it a fautly tranlsation (though over the years, I have read different ones and had the same question) ? Does Tolstoy tell us, or are we to infer that the Countess had eight children that did not survive infancy (& would not the death of Petya bring forth those bitter memories?)

Thanks for clarifying a question that has literally pestered me for years!