Unfortunately, as Stef mentioned, we have much more flights than aircraft that could fly those. Speaking about number of aircraft means the Fleet size mentioned on the Fleet download page. Basically, in the good old days the Fleet size matched with available flights. But again, not in high classes...
As mentioned, EHM owns 12 Class1 aircraft. This means (momentary forgetting KATL) 2 per HUB, which is not so many, but quite real when you check available Class1 flights. Schedule of these aircrafts is not really tight - I also do not remember if I ever seen an EHM-AI Beech 1900 anywhere, because they are always flying somewhere. The situation is getting worse with higher classes, and terrible at Class7. We have (momentary forgetting A380) 60 Class7 heavies, but if I remember well I once calculated we would need almost 200 to cover all available flights in the timetable.
All these means, EHM B747s and A330s and A320s are blocking the gates, if the AI timetable matches with our real timetable. It does not look so bad at EHAM, where there are a lot of KLM planes. It does not look so bad at EGLL, where there are a lot of BA planes. But I use very dense AFCAD files, and I do not have a clear spot neither at Heathrow, neither at Amsterdam, nor at Lisbon. If I think around 1/3 or 1/4 of EHM planes should be there, certainly there would be empty gates for other AI, or being really empty. On the other hand, does not matter how accurate AFCADs I have for smaller airfields, EHM planes never pops up there, even if there are flights to that airport.
So, afterall HUBs are overcrowded, but small airfields are empty with this "unreal" AI timetable. The size of the timetable is a little bad for AI flights, but really nice for us, the pilots. It would be really nice if you could provide your AI timetable, because it is a huge task to make it, regardless the reality level.