Buying 6x pcie slots mobo is like a waste of money to me, I thought I would stop upgrading my rig when I have 6 graphic cards running but now I have enough money to add more cards, what comes to my mind is
1. What have I done? Why buy mobo with 6pcie slots when I can afford 12x pcie slot mobo in the first place?
2. Why buy 750watt power supply when I can afford 1200+ power supply?
Now I have to buy another MOBO which will result in more money and more electricity consumption too, if you are new to mining always go for the best components just in case, there is a chance of upgrading in future
A modern motherboard with a processor consumes 40-60 watts, and you will not save much money if you buy 1 motherboard for 12 video cards instead of 2 motherboards for 6 video cards.
Further, there are many nuances about power supplies. Even on an ATX 1200 watt power supply unit, there may not be enough key connectors to connect 6 powerful video cards, so you have to use 2 850 watt power supplies or a server power supply unit.
It's harder to find a buggy graphics card on a farm of 12 graphics cards instead of 6.