would 2 x 750 PSU be enough for 6 x RX580 8GB rig?
also this have been bothering me for a while. might be a stupid question but don't laugh. what does "on/from/out of the wall" means?
Yes, two 750 watts PSU's should be fine as you should be around 900-950 watt total system draw.
I assume you are referring to a rigs power draw when you refer to "at the wall". It usually indicates the user is referring to the fact they are citing figures measured using a power meter placed in-between the PSU's AC input and the outlet itself so as to measure the true total system draw. This is different than others who may cite simply the power reading obtained from software, such as GPUz and its power draw reading. The problem with software readings is they are usually incomplete and only measure part of the total consumption.
Since an "at the wall" measurement includes all the components of a mining rig (GPU, CPU, RAM, Mobo, PSU, HDD or SDD, etc.) you will get the most accurate figures for determining your net profit if you pay for electricity.