It is not clear how many Forgotten Ones there were/are. We have been given the names of 3 of them but that doesn't mean there weren't more, simply that their names have been forgotten by the Dalish. From what I recall we didn't get their names from Dalish Keepers but other sources. It was Felassan, an ancient elf, who identified one of them as Anaris; Daern'thal was mentioned in a codex about an item and Gelduran was giving his views on the elven gods in a codex in JoH.

So there could have been others. It would make sense really for them to be an equal number to the Creators, hence the reason the battle between them was so evenly balanced that, according to the Dalish, both sides were willing to call a brief truce whilst he negotiated. Whilst the Dalish story of the war between the gods may not be entirely accurate, nor do I think it was wholly wrong. Gelduran shared the view of Solas that the Creators were not really gods. Clearly at some time he had been banished by them or fled from their influence. According to the codex in the Temple of Mythal the Forgotten Ones hung out in the Void. Whilst the Dalish claim that Fen'Harel was able to safely walk in the camps of both sets of gods. Could this be because he didn't go there physically but contacted them via the Fade?

If the number of Forgotten Ones corresponded to the number of original Evanuris (7) since Ghilan'nain and Solas were late additions to the pantheon and subordinates to two of the original gods, then it is entirely possible that the theory about the discs around the circle containing the idol are correct and they are connected in some way to maintaining the Veil and keeping the Evanuris imprisoned. Solas says that foci were a means of channelling power, so if each of the Forgotten Ones had one, may be all 7 were required to power the spell, with Solas providing the central focus. Then just as Solas fell into a millennia long slumber to recover his strength, so did the Forgotten Ones. Hence the old god dragons seemingly being in hibernation.

It would certainly explain why Solas was beside himself at the prospect of the Grey Wardens seeking to kill the last two old gods if they were all that is left of the team that imprisoned the Evanuris and he was not yet strong enough to face his old enemies. However, there are a few problems with this theory:

He claims that if Corypheus had died as he should, Solas intended recovering his orb and tearing down the Veil. He made it sound like he would have done this straight away but he was still weak, so that would make no sense. So may be all that was required was to unlock the orb and there would have been a time delay anyway before he felt ready to put the next stage of his plan into action.

Darkspawn are drawn to the location of the old god by its song, which the Grey Wardens can also hear. This suggests that the old god is already an arch-demon before the darkspawn reach it as it is a corrupted song that they hear. In which case, it is more likely that the corrupted dragons are the split soul other half of each of the remaining Evanuris/Creators and Solas imprisoned them in the Deep Roads at the same time as he shut their other half behind a barrier in the Golden City. That being the case, the discs around the circle may simply be representative that only two of these dragons are still alive. If they were killed and their souls allowed to return to the originator in the Fade, may be that would give the Evanuris the required power to break free of their prison without action from Solas. In this scenario it was the Evanuris whispering to the Magisters from the Fade, encouraging them to go to the Golden City to start the process that would lead to their release. The dragons were already infected with red lyrium before being shut in the Deep Roads and it was the desire of the Evanuris to use red lyrium as a weapon and Mythal forbidding it, that led to her murder.