I've rent these at least 6 times. there are as one might expect, a large van.
The one down side is that the seats don't fold down at all, so packing bags in can be a bit of a pain. They drive well, but you need to allow slightly more room in making turns.
It looked to me as though a further set of seats could have been attached to the back of the vehicle but they had either been removed for our benefit or removed completely to make into a permanent 12 seater. I would imagine that once those seats are in place you have very limited luggage space.
Any one we hired the last row could be taking out to make it a 12 seater its been a few years now since we hired one The time we rented a 12 seater it was a 15 seater with the last row out
we have already booked a 7 str, but dh wants to change it for a bigger van as we will have dd, partner and 2 kids and also his sis and family, they already have their own cars but we can all get in the bigger van for the odd day out together.