Does interacting with a part of a complex illusion makes you aware that the whole thing is an illusion? For example, if you creata an illusion of a forest with hallucinatory terrain, does one only need to interact with one tree to dibelieve the whole illusion?
Yes. One illusion - one save.
I agree, spells are generally treated as single entities unless called out otherwise.
If you want to force someone to make 20 saves to see through your multi-layered illusion you have to actually cast 20 spells.