Flamingoland has a +1 carer maximum, Efteling it varies per ride (so Flying Dutchman is +2 carers, others +1, some +3, no disabled on Dreamcred), but it depends on the person in question... Sure some would need 2/3 carers to ensure safety throughout (plus I think the numbers is more to aid with evacuation procedures as well, as in some cases 1 would simply not be enough) though so it's hard to pin-point an exact number...
Saying that, I always used to turn a blind eye to groups of 5/6, but at the same time the amount of people I had to explain how it worked to was ridiculous... Always a lack of communication on these matters...