Statistics: How Many Flamers of Tzeentch?

Flamers of Tzeentch are known to be a "glass hammer". That is usually meant in the sense that you'll get one hit out of them before they disintegrate (due to enemy fire!).

So, what role do I have for flamers on the battle field? There's two broad roles really. Firstly: tank hunting, and secondly: elite troops hunting. Combined with their jump infantry status, they should be able to let rip one volley before perishing horribly. So let's have a look at how they do.

Anti-Tank.
Their main weapon is the template attack. It will automatically glance anything with an armour value on a 4+. Take two flamers, and I will statistically glance once. To get a reasonable chance of a high glancing result, about 6 flamers will suffice.

Anti-Elite Troops.
When people think of elite troops, they will often think about terminators. But plague marines are also a kind of elite troops due to their quasi-terminator saving throw and high toughness.

In one of the Warpstone Flux contests a while ago, we saw that flamers of Tzeentch were simply excellent against plague marines, even in small numbers (poetic justice or what?!).

Against terminators, they fare slightly worse due to the 5+ invulnerable save that the terminators have. Even so, a small squad of flamers can really take the fight out of a squad of terminators due to the breath of chaos. The statistics are a little hard to work out given the vagaries of template weapons, but from play experience, slightly less flamers than the terminator squad will be needed (i.e. about 4 flamers is often enough to take out or severely weaken 5 terminators, assuming optimal flamer template placement).

Survivability.
How many bolter rounds can flamers of Tzeentch take? Fired by marines, standard bolter rounds hit on 3+, wound on 4+ and kill on 4+. That's 0.167 dead flamers per shot; or 6 standard shots to down them. So in some ways, yes: they are something of a glass hammer.

Summary.
Even 3 flamers can pose problems for tightly packed marine squads. Taking a few more for anti-tank and anti-elite jobs ... and I think that 4 or so flamers is about the right number.