not sure where the reference is, but i'm pretty sure (95%) it's only one per unit in the shooting phase, but one per model in the assault phase (assuming of course it's a target you can use grenades against). Key word being thrown, in combat the grenades are not thrown, but attached.
My guess would be they refer to phase because you could throw a grenade in the assault phase as part of overwatch (and would therefore also only be 1 grenade per unit)
_________________
Tan? You're joking, I'm a gamer, you're lucky I'm wearing deodorant!
My Blog - The Burning Eye Blog (check it out - comments always welcome)
My Project Log - Visions of the Burning Eye
My Gaming Log - Chronicles of the Burning Eye
My Club - MAD Wargaming
My Fluff - Kabal of the Burning Eye, Cult of the Shadowed Blade and Coven of Distorted Perfection