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?

Depends if you dual mine or not. I have a 6 card rig with 4 RX 480/580, 1 RX 570 and one 1080. I use an EVGA 1000W P2 with 6 PCI-E connectors for the VGA power inputs only and use an EVGA 750W P2 for the motherboard and all the risers. Dual mining the VGA's draw ~805W and 360W for the motherboard and risers. If you dual mine, you might be able to get two 850W to work, but I would go with the 1000W so you have enough PCI-E connectors for the cards.