If I run all 12 graphic cards on a single motherboard vs using two separate motherboards (6/6) will I be able to safe extra power? A motherboard takes up to 120watt depending on the CPU and gen type that's why I think this is a reasonable question to ask .
Aside from saving power you could also save space, this is recommended if you have the existing mother which can accommodate up to 12 gpu. If you are still planning to buy another mobo to use it for 6/6, better buy it with a close mining frame, a good frame is security for your cards from outside disturbance.