You will have to test, to make sure whatever you decide on, works. The VRD has much better scenarios than I can write about here.
In an open space, RF can propagate very, very far (300 meters), so whatever you broadcast will be seen by everyone and could possibly interfere.
Regardless of what everybody thinks, unless everybody is taking a test via Wifi, in an auditorium, not everyone is using wifi at the same time. You can safely think that from a capacity perspective, 1 out of 5 users could be using the wifi. Speak to your customer to get a good idea of this. Of those 1 out of 5 users, they have well-behaved applications that are designed for mobile like web and email that can tolerate a great deal of congestion.
If you have to deploy more than 4 access points, you might consider turning off one or more radios on the 2.4 ghz side, so that there is less interference (you can also turn on ARM-Aware on the 2.4 ghz side so that you can also get air monitoring)
You might also try turning the power all the way down for all those access points, because even at the lowest power, RF propagates pretty well in open space.
You also might try using the Cell-Size reduction feature so that access points will ignore any energy outside of a certain radius.
Make sure that you only use 20mhz channels, even on the 5ghz side so that you get good channel reuse.
Try to schedule a test with a temporary setup with a bunch of volunteers before you deploy.