In the constant search for realism it is tempting to activate more than three AI aircraft. This can lead to all sorts of troubles with ATC.
Apart from your own aircraft there are three other possible types of aircraft found at an airport:
Static aircraft – These are simply non moving models and use very few resources. The just make the airport seem fuller.
Traffic aircraft – there are a number of traffic addons available – both freeware and payware which are controlled by the traffic add-on they are paired with. They move, taxi, take off and land but they are actually not connected to the X-Plane ATC system at all. They have relatively little effect on X-plane’s performance.
AI Aircraft – These are aircraft that are controlled by X-Plane’s ATC – with ever increasing intelligence. They are essentially the default aircraft of X-Plane itself. They even talk to ATC. The problem is that each additional aircraft uses a lot of extra resources and this extra load first becomes noticeable when ATC has long pauses before giving you instructions. Too many active AI aircraft also make conversation with ATC difficult because there is much more chatter, and you have to find a gap to send your message.
One or two AI aircraft should suffice with an absolute maximum of 3.