Thanks for the reply. With your hint and further research, I've figured it out.

The vans I was going to rent from Sixt said 12 passengers, which were Starex or Grand Carnival or equivalent. I googled those vehicles, they don't exist in the US, and they looked like minivans so I assumed they fit 7 and not 12. 12-passengers vans in the US are MUCH larger than these vans. So after you suggested Starex, I looked into it more carefully and it turns out they actually do fit up to 12 ppl. Pretty insane for that size. Then I dug into Grand Carnival as well and they have a 11-person configuration. Looking at the seat map, it looks super cramped, like a van I've ridden in South America or South East Asia. Well, these must be the same ones. https://www.kia.com/worldwide/vehicles/carnival.do

So... I booked one of them.