Why so many jump seats [on the Ironclad Assault] if the crew is only six?

Because people get knocked around when you're going into atmosphere, to be blunt. It was somewhere so that you don't have to sit inside your tank while you actually land, and you can go sit in the jump seats and not get thrown over.

And the answer is it's not for the crew, it's for the people you're transporting.

Yeah, it's for your guests.