There's only the conventional way of transporting casualties: One patient per ambulance. But incidents are limited in terms of patient amounts, as far as I know there is a maximum at about 20 casualties. And that occurs rarely.
Well, you have to decide which special units you can implement with the given types. In lots of EOCs volunteer stations or groups with additional ambos can be called for assistance. Some owners use "FR" for non-transport vehicles (commands, material support units, personnel carriers and more) to simulate full disaster response teams. They will call themselves available if there's no patient without treatment on scene, though. So players need some fantasy for keeping them in status 1 until the situation is cleared.