I would use a bunch of EAP320/EAP330 with high wireless throughput in order to provide AirTIme to as much people as possible. In theory, you would need at least 8 or so EAPs to handle 400 clients, but in practice it a) strongly depends on the surf pattern of the clients and b) will cause interferences in the 2.4 GHz band, since you only have 4 non-overlapping channels in 802.11n mode and only 3 non-overlapping channels in 802.11b/g modes. Are all 400 people really on-line at once?

We once supplied a big happing with WiFi and there were ~3.000 visitors, but only 100 to 150 actually using WiFi in the break room at any given time. We used four dual band WiFi routers configured to different channels, 20 MHz channel width and low TX power to have each AP just supply one of four quadrants of the area without disturbing the signals of the remaining APs. If you want to set up such a topology, you should measure signal and SNR ratio of the devices before-hand with a professional WiFi analyzer such as NetSpot.

Perfectly answered by winstar.

No, I would not just recommend 10 EAPs without having measured the effects of installing more then 4 devices in the room b/c of congestion of the 2.4 GHz band. That's what I tried to make clear with my reply. You can probably handle up to 2x30 clients with one EAP and possible some more, but since the 2.4 GHz is not only limited, but uses overlapping channels, just increasing the number of devices will not help much. Keep in mind that at any given time only one device can use a (non-overlapping) WiFi channel. So if you use 4 non-overlapping channels in the 2.4 Ghz band (possible with 802.11n), then only 1 out of 100 users can allocate the channel at a given time,a ll others are silenced until they get a free slot in time. This sets the limit.

Then the EAP330 with band steering and antenna beamforming would be the right choice to support as much as 50 to 60 clients simultaneously, maybe even more.