2 per army, armed with Heavy Bolters and Missile Launchers:
Fluff wise this revelation bodes well for the Iron Helms. Especially the description "As the teeth of the World Eaters, Devastator Squads rarely limit themselves to tactical support. They are often found in the thick of any fight, taking blood with as much abandon as their comrades in arms"
This coupled with this note on the Iron Warriors (found on the Lexicanum website) "They are also fierce close range fighters when they must come to grips with the enemy, witnesses comparing their ferocity to the berserkers of the World Eaters traitor legion or the loyalist Blood Angels.", makes for an interesting chunk of back story for me to build upon
HOWEVER! being fairly stuck in my bloodthirsty ways, and a firm believer of "If you can't use it in close combat then it's not a real weapon..." I am rather short on Missile Launchers. In fact I only have one, here he is blowing something up:
I do however have a fair few Heavy Bolters. So watch this space...