Ok so I tested my DIR-857 Rev A1 v1.02, Mixed ALL modes, Mixed G and N and N only, Auto 20/40Mhz, Channel 11, WPA2 and AES only and i'm able to get 7 devices connected on 2.4Ghz all at once:
Started with 5 on 2.4Ghz and one on 5ghz to monitor wireless status:
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6 Devices connected on 2.4Ghz:
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7 Devices connected on 2.4Ghz:
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The only thing that I noticed is that one device with a DWA-131 does not connect using single mode N for some reason, this maybe a DWA issue not related to the router as you can see it's the only device connected in G mode. Other then that, all 7 connected and internet or network testing passed.
Devices used:
Mac Book Pro, iBook G3 using a DWA-131, 2 iphone 4s's, 2 PC's using DWA-160's, and a xbox 360 using a 2.4/5Ghz wireless adapter.
I hope this will let D-Link know that the DIR-857 supports more than 6 devices. I presume the DIR-835 should as well.