There are 8 Unique Planets (Excluding the Major faction Homeworlds) + Harrow (The Academy)

They are: Tor, Kyros, Auriga, Bilgeli, Sahar, Sykagoja, Veil & Teonha

The number that appear in a game depends on the galaxy size with only 1 on tiny going linearly to all 8 on Gigantic

They are currently two extra unused unique planets: MU3R74 & Chiark

MU3R74 Description: From space the planet seems typical of many an advanced civilization's--large-scale infrastructure, dense urban/industrial areas, cultivated wildernesses--except for one anomaly: the complete absence of a comms spectrum at any wavelengths. Planetside, the truth becomes shockingly clear. MU3R74 is a ghost world, terraformed for future inhabitants who never arrived. The corroding shells of Sowers, the self-replicating robotic workers tasked with making planets habitable for the Virtual Endless, litter the world. What befell the Sowers is unclear. Machine plague? Mass suicide? Whatever it is, the outcome looks like a silent apocalypse. 

Chiark Description: Whoever shaped this world into its present form is lost to the sands of time, but it is clear they possessed a mastery of time and space--and games. The whole world is, in fact, the galaxy's most complex game, and players can spend whole lifetimes attempting to master its intricacies. Shrouded in thick mists from the outside, players who "enter" the game find themselves in something akin to a vast living simulation and must rely on all their wits and knowledge to progress. What purpose the game serves is unknown, and whether anybody has ever won is arguable, but what is certain is that the world attracts the galaxy's best minds.

thanks for the info!