Lovely to hear you have hedgehogs. It sounds as if they have you right under their paws!
It isn’t possible to tell how many hedgehogs are visiting by how much they eat. What we feed them is supplementary to what they find to eat in the wild, so that some hedgehogs will inevitably eat more of the food we offer than others do. Even if you were able to see how many hedgehog visits there were a night, some of the visits could still be one hog visiting several times. Although it is possible to learn to recognise individual hedgehogs by their natural markings, which is something you might want to explore.
Hedgehogs are a bit inclined to like sticking to the sort of food they have become used to. So that if you mix it with another kind of food, they are likely to show a preference for the original food. If you were to give them only the alternative food (and it was suitable) they would be likely to eat it eventually if they were hungry. Although they might ‘go on strike’ for a few days to start with. The best time to introduce a new sort of food is just after hibernation, when they are hungry and likely to eat what they’re given.
Make sure you always leave a source of water out of the hogs, all day every day, just in case a dehydrated hedghog should come along. Large, wide but shallow plant saucers are ideal for water.
Good luck. I hope you continue to enjoy the hedgehogs’ visits.