I uh... I'm from that thread.
SoC is, like you said, at LEAST 9x baseline Star level for containing the souls of all confirmed Lords of Cinder (Lord Gwyn, Solaire of Astora, The Chosen Undead, The Bearer of the Curse, Yhorm the Giant, Ludleth of Courland, Aldrich the Devourer of Gods, The Abyss Watchers, and in theory Prince Lothric (although Prince Lothric is questionable as some theories do state that he did indeed begin fueling the First Flame, but... grey area and whatnot). There is also mention of other souls (Vendrick is said to have made use of a particularly powerful soul to keep the waning of the Flame at bay for awhile whilst he researched a more permanent cure- there's no mention of how long and considering Vendricks long life, this could have been a very long time indeed). It is viewed that it could have been Vendrick's soul, as he went Hollow shortly afterwards. I don't care much for DS2, so my knowledge is limited.
So, you can safely say there are at LEAST 9 LoC souls powering the Soul of Cinder, but... the potential for more is incredibly high. Fire Keepers tend to Chosen Undead, those who would become Lords of Cinder. There are hundreds of said Fire Keepers at the DS3 Firelink Shrine alone. There are many firelink shrines. I would say, upside, there could be over one thousand total Lords of Cinder contained within SoC, but... again, only estimations.
So he's pretty powerful.