The NPC itself only has code for six NPCs to be dropped off by it. You could probably work around it if you were making a map, but as far as I am aware, Valve never used more than those.
The container's in-game model has a surprisingly small space inside when it's open. It only goes about half way back into it:
However, this could (rather comfortably for Combine standards) fit at least eight:
If we assume the rest of it is hollow inside that the model doesn't show, this could probably store at least 20 with room for some gear:
I didn't try to pack them as tightly as possible, so if you were just cramming people inside it could likely pack in 30, but it'll be very cramped.