Originally the forerunners built 12 30,000km wide rings on the less secret greater ark. That ark and 11 rings are destroyed before the array fires. In the process of building those they learned they could make the rings 10,000km wide and they would only need 6 of them. Far more efficient and logistically much easier to move and build. These are the rings you interact with in the games and the smaller and more secretive ark is the one you see.

TL;DR the tech got smaller and more efficient over the millennia it took to build the first 12.

I just pretend the series ended at 4. The Greg bear and Karen Traniss books are good too.

All six of the smaller ones made by the lesser ark from halo 3 as well as one bigger older ones fire about 100000 years before the events of the games. The 19th halo, installation 08 that was being built to replace 04(that we blew up) also fires independently destroying its self(because incomplete) and is replaced by 09.

So one array fire and one incomplete solo fire both to wipe out flood infestations.

Pharaoh ants.

Two, put your junk in that box