Managed to put 6 RX 480 on a mobo using Corsair RM1000i 1000W. Running fine for an hour or so. Will update if my rig shuts down.
P/S: Did not test it with watt meter
Setting: Stock Memory, GPU & Power, 60~80 Temperature, 2000~4000 Fan Speed
Rig: Intel Celeron, 8GB RAM, 1TB HDD, MSI Z87-GD65
My undervolt 6x R9 390 draws 1030W from the wall and hash at 165 MH/s. I use the 1200W Corsair platinum.