Assuming we’re talking just, like infantry Marines, and what they can carry, and not implying the level of naval and infantry support they would be normally receiving, I can’t see less than like 500 million being able to take the planet. I mean, they have a fairly powerful short range weapon, and armor the can survive the majority of handheld small arms. That’s not massively impressive. There is a low intensity war going on right now where dozens of tanks with superior armament, speed, and armor are being destroyed every single day. I don’t care about the hardness of the fantasy metal, a javelin or Maverick or moderately sized bomb is absolutely cooking alive a space marine. Any attempt at open combat would be met by a longbow Apache, barely even within visual range, launching hellfires until it’s out of ammo and has to be relieved by the 40 Apaches behind it. Most attempts of stealth or subterfuge would be foiled by them being goddamn massive genetic freaks, unable to blend in in any society. You would need an apocalyptically massive army for them to constitute an apocalypse..