In each expansion, there is a series of 3 alliance raids (24 people). There is also a series of 3 sets of 4 raids (8 person), for a total of 12.

In ARR, these are Crystal Tower and Coils.
In HW, these are the Mhachi and Alexander.
In SB, these are Ivalice and Omega.
In SHB, these are Nier and Eden.
In EW, there is only currently the 8-person raids from Pandaemonium.

The Firmament has nothing to do with raids and is actually all about crafting and gathering.