When 2000 Yielothaxes die and player kill the Raging Mage you will see a raid message:
The remains of the Raging Mage are scattered on the floor of his Tower. The dimensional portal quakes.
It means that the boss was killed and there won't be possible to face him till the portal to other dimension will be open again. So in fact we've got only one Raging Mage per day. It is possible to kill him by several players, but all of them must to fight with him at the same time and only one person is able to loot Elemental Spikes, so the most often players simply do everything to kill it solo.
If any player come after some time - the place where boss spawns will be empty till new portal appear.