The 802.11 protocol has a hard limit of 2007 associated clients per AP (per BSSID, per band), because of limited number of Association IDs (AIDs) that can be represented in the combination Duration/ID field in the 802.11 header. That Dell box you linked to appears to have a single radio, and unless it can be configured to support multiple BSSIDs (that is, act as multiple virtual APs), it probably has a hard limit of 2007 simultaneously associated clients.
Even boxes that allow you to configure multiple BSSIDs don't usually let you set the same [E]SSID for more than one BSSID, so it's typical to have a hard limit of 2007 simultaneously associated clients per SSID per AP.
And of course, even if all your 2007 clients could do the same 600 Mbps 4 spatial stream 802.11n that that Dell box can do (which is pretty rare; the vast majority of 802.11n clients can only do 300 Mbps or less), and even if they were all packed in close enough to maintain that rate, if they were all trying to move traffic at the same time, they'd each get less than 300 kbps, which is a pretty crappy rate. So your practical limits are much lower than the protocol hard limit of 2007.
In the past, I've seen 802.11n AP radio hardware that only had hardware support for 50 or 64 client WPA/WPA2 keys. Beyond that, any additional clients had to have their WPA/WPA2 encryption done in software, with a significant performance penalty. This is just one example of a class of implementation-specific practical limits or hard limits that may exist in any given 802.11 AP.
I heard somewhere that there is 255 device limit. Is that true? – Mr.D – 2016-06-09T10:02:20.207
@mr.D From a hardware perspective: No. But it is possible that the software that comes with that Dell device imposes that restriction somewhere. I never seen one of those so I can't say for sure. (I answered based on general knowledge that always applies to such devices.) – Tonny – 2016-06-09T10:09:48.380