Seeing just how many AI/computercontrolled planes will run in the latest version of PicaSim (under Windows). "Quite a lot" seems to be the answer.

http://www.rowlhouse.co.uk/PicaSim/

Support for AI controlled planes is also in the Android version (though for memory reasons limited to about 40 planes, rather than the 100 shown here), and it will soon be in the iOS version too. The actual number you can have depends on your device low end devices will only support a few before the frame rate is adversely affected, but high end devices will support many more.