[Suhiir]

Ideally whatever vehicle is adopted will carry around 15-20 passengers, a standard 13-man squad plus a 2 to 4-man support team (AT, MG, Engr, whatever) that's a pretty much standard attachment to a Rifle Squad.
The real problem is the USMC can't use large numbers of vehicles simply because all the vehicles used to transport a landing force have to be able to fit into the US Navy amphibious ships. Unless a vehicle that carries half as many troops is literally half the volume and mass as one that carries twice as many it just plain won't work. This is why USMC LVTs are so damn big in the first place. Their APC function is secondary to their amphibious one.