There were in fact 12 original rings, but most of them were destroyed prior to the Halo timeline. In-game, only seven remain, Installations 01 - 07. This does not include The Ark, which is considered Installation 00. In Halo: Combat Evolved, you fight on Installation 04, which is effectively destroyed when the Chief detonates the [i]Pillar of Autumn's[/i] reactor core. It is revisited in Halo 2 as the Arbiter travels there to kill an insurgent group that was stranded on a gas mine after the events of Halo 1. In Halo 2, you fight on Installation 05, which is rendered inoperable by the Covenant Separatists (the Elites) after the Arbiter killed Tartarus and Keyes deactivated the ring as it prepared to fire. Since the Flood was spreading uncontrollably, the Elites glassed its surface. It is discovered that the ring's near-activation placed the remaining 5 stations into stand-by mode, whereby they could all be activated simultaneously from a remote station (The Ark). In Halo 3, you fight on Installation 00, where the Covenant is attempting to fire all the Halos. It is also discovered that Installation 04's destruction triggered the activation of an automatic repair sequence, which was effectively rebuilding the destroyed Installation 04. Both the Ark and new ring are destroyed by the Chief when he fires Installation 04B (as it is known). After the Human-Covenant War ended, the UNSC [i]Infinity[/i] was created to hunt down the remaining rings and deactivate them. In Halo 4, you fight on a station that was created to study and secure Installation 03, which is briefly seen in-game. The Halo Array was originally created by the Forerunners to be a last-ditch desperate measure against the Flood. Upon activation, it annihilates all sentient life in the galaxy, which the Flood needed to survive. Thus, the Flood-Forerunner war ended in a stalemate when the array was activated, destroying all life in the galaxy. There were DNA samples saved of every single creature in the galaxy, thanks to the Librarian. After the array was fired, the index of DNA was released to "re-seed" the galaxy, essentially starting everything over again. These DNA samples were saved on the Ark, which was outside the effective range of the halos. There were also other ways to avoid annihilation; a number of "shield-worlds" were constructed to preserve life, and some of the halos themselves had preservation methods, which explains the continued existence of the Flood within them, which were kept in stasis for study purposes.