By the power of truth, I, while living, have conquered the universe.
In Cryptum, we're told that there were originally 12 Halos created under the orders of Faber when the Builders took political power in the Ecumene.
One of the Rings are stolen by Mendicant Bias, he fires it at Charum Hakkor and the Timeless One stows away on it. Then he assaults the Capital and a battle ensues to destroy the Rings, however we only have half an account of the events because Bornstellar leaves with Glory and a Councillor.
So I'm here to examine the possible fate of the Rings.
Firstly, I've seen people ask why the Ark didn't simply rebuild them up to the 12 there originally were. As we know, in the games, there are 7. The plan (as in the plan) to destroy all life in the galaxy hadn't been sanctioned at the time. They had built the Halos as research stations firstly, designed to contain living specimins for indexing. Thus, it wouldn't have been prudent to rebuild the Ring at the time.
Secondly, it's likely they had no idea whether to rebuild it or not. The construction of the 12 Rings was initiated when the Builders took power and cast down the Warrior-Servants (the ones who proposed the Shield Worlds); you'll remember that Faber, the Master Builder, was removed from power by the Council for firing a Halo at Janjur Qom to quell the San'Shyuum revolt. That was the tipping point where the Builders lost power, so the new Council may have changed the rules.
An interesting bit of speculation here, but what if the remaining 4 Halos were not destroyed? We know that Mendicant Bias can control 5 Rings at a time, what if he stole them at some unknown point? We know that Primordium is set on the Ring that Bias stole and hid the Timeless One on, so what if he managed to (at some point) nick the others?
Having recently reread through Cryptum to formulate this, I came across something on page 329.
"The Falco slowly rotated and a complicated silhouette moved across the spiral cloud. It took several long moments before I could make out slender shapes attached to that silhouette, like a wide rosette. Slowly it dawned on me that I was looking upon another array of installations: six rings, each rising from one of the petals of an enormous flower.
We know that 1 Ring survived, and here Born is telling us that there are 6 being constructed on the Ark. So it's evident that these are the Rings used in the final plan, it's a logical conclusion that (since we know little of what happened at the Capital after Born, Glory and the Councillor left) the other Rings were destroyed or went missing. Also, it says during that battle that only 7 were being recalled into slipspace where only 1 survived - so it's clear that only 7 either needed to be used or were planned to be used.
I hope this brings some kind of closure to people who didn't quite understand what was going on.