The Nine Homeworlds (also called the Nine Gallifreys) project involved the cryptoforming of eight planets into eight cloneworlds of the original Homeworld. It was spearheaded by House Mirraflex's Lady Armourer Mantissa and House Lineacrux.
Officially, each Homeworld was run by a caretaker House, and could be used as a replacement if the original Homeworld were destroyed. But they were also supposedly meant to act as decoys. But since the original Homeworld was "obviously" in the same place it always had been, they could only work as decoys if the duplicate Homeworlds each had a War King or War Queen and thought it was the original, and one of them had replaced the true original. There was a supposedly-total ban on contact between Homeworlds, but Mantissa eventually went into exile on one of the duplicates, further reinforcing this. Regardless of which Homeworld was the true original, the secondary Homeworlds (believing that they were the primary) would create tertiary Homeworlds (which they would believe were secondary), and so on until the universe could eventually have become nothing but Homeworlds. (PROSE: Nine Homeworlds, Lady Armourer Mantissa)
The War King (PROSE: War King) became Lord President (PROSE: The Taking of Planet 5) of what purported to be the original Homeworld, six years before the War, and continued for over half a century. (PROSE: Timeline)
Two years before the War (PROSE: Timeline), when Compassion became the first 102-form timeship, Romana was also War Queen (and "Mistress of the Nine Gallifreys") on what seemed to be the original Gallifrey. (PROSE: The Shadows of Avalon) This Gallifrey was invaded by Faction Paradox before the destruction of the Eleven-Day Empire, (PROSE: The Ancestor Cell) who claimed to have written the other Gallifreys out of existence. (PROSE: The Gallifrey Chronicles) During the invasion, the Gallifrey was destroyed. (PROSE: The Ancestor Cell)
The incident where a Homeworld was first lost was called the battle of Mutter's Cluster. At the time, this brought the official total down to nine. However, it was rumoured that there were even more hidden in pocket universes, and that even the War King didn't know which was the original. (PROSE: The Taking of Planet 5) Homunculette was trained in escapology on Gallifrey XII. (PROSE: Alien Bodies)
More than fifty years into the War (PROSE: The Book of the War), Gallifrey Eight contained loomstacks that worked day and night weaving soldiers, throwing biodata soot into the air. It also contained chronoforges to create weapons for use against the enemy. (PROSE: The Taking of Planet 5)