1: 7 Installations
2. Cpt Jacob Keyes
3. Reach
4. 343 guilty spark
5. Spartan II

1. 7 installations
2. captain Jacob keyes
3. during the final days of the fall of reach the pillar of autumn landed on reach to pick up an advanced AI named Cortana, shortly after it landed the pillar of autumn is attacked by the covenant hoping to seize the AI before it can bee brought aboard the ship. noble team, a brave group of spartan III's sacrificed themselves to deliver the AI and cleared a path through the covenant blockade for the autumn.
4. 343 guilty spark was in charge of installation 04
5. John 117 nicknamed the master chief is a spartan II

there is 7 installations including three more just like what boopjack said also the ark (installation 00) and the lesser ark. how ever keep in mind in halo 3 the lesser ark all ways had a halo almost ready in case of an emergency. but there is proof of 12 or more halos not all of them are not discovered. halo rings range is 25000 light-years the milky way is 52,850 light years in real life (as far as we know) and both arks are at the edge of the galexy there is possible there could be more ark and if they do the same thing as the lesser ark then they will be more halos as well. so right now every one could be right

Ah, I should've specified how many active rings during the games (not counting the replacements or the original Greater Ark rings that got destroyed). Still, too late to change it now. Either way, thanks for pointing it out. I made this in a rush years ago for a school project, so I was looking to type them out quick and get 'em on the box.

There were originally 12 rings, actually.

I never realized that the Ark in H3 isn't THE Ark from the lore. TIL!

By how many were built, I meant how many original rings, I wasn't counting the ones that came during/after the Human-Covenant War. Still, I should have been more specific. Thanks for pointing that out!

quick lore fix, there was only seven halo rings but there was nine halo instalations(the last two being the ark and the lesser ark(the lesser ark being the one you go to in halo 3))