One of the many Gallifreys existing during the War in Heaven was lost during the Battle of Mutter's Cluster, bringing the official count down to nine. (PROSE: The Taking of Planet 5)
Behind the scenes[]
Within the novel The Taking of Planet 5, it was altogether unclear whether this Gallifrey was utterly destroyed, or if it was "lost" in a similar way to Ardethe or New Alexandria. The nobel was also unclear as to if this "lost gallifrey" was a duplicate, or if it was the original previously mentioned by Homunculette in Alien Bodies.