A RX 580 properly tuned meaning under-volt mod applied with clocks and voltages properly adjusted will draw 130 watts when mining ETH only and ~150 watts if dual mining. So for ETH only we could do 5 x 130 = 650 Watts, dual mining it would be 5 x 150 or 750 watts, well within reason. Even if he went 6 GPUs (which I only speced 5 for the AMD RX 580, the 6 was for Nvidia 1060's) it would be 780 and 900 watts respectively. While a 6 x RX580 dual mining configuration might be starting to push the PSU, it is still well under the maximum. However, a 6x RX580 just ETH only would be well within reason at 780 watts, or under the 80% load guideline I like to run around.
The key is as you said as i said is : " properly tuned meaning under-volt mod applied with clocks and voltages properly adjusted"
So do you think when someone asking a question like : "I dont have the slightest idea how will i build one " , that person know how to properly tune , mod the cards ?
PLease ! dont give bad advice for newbies who dont have "slightest idea how he need to build ", they can burn down they house !
I have SEVERAL rigs of 7 x Rx 570's running at ~115W Dual mining, for a total system draw of about 850W on 1000W platinum PSU's so ... I agree with Za1n, if your'e getting into mining, you need to learn how to optimize, or you stay away.