scrypt mining rig suggestions
What i'm wondering is how many PSU's would I will need and what kind is recommended? 


I would recommend much higher PSU, at least 1000 Watt PSU for each two Radeon's R9 290x. This is based on some recommendations I found (here and here). I am 99.99% confident a single 750 Watt PSU will NOT be able to cover 6 GPUs of that power. 

I also think the motherboard you selected can only take 3 cards, you need to double check. Even with 3 slots the cards may not have enough room between the slots, so you will need to use extension cables. The CPU is actually overpowered, I would select a low-power one. CPU will not be used for mining. Memory is not used as well. You will be covered with a 2-4GB stick to run the OS processes.

No one seem to be bothered with a case. For that much price of a hardware you don't want much dust to come in. Or any morning coffee spilt on your expensive hardware... I would advice to buy a large, solid, enclosure with good ventilation. This would cost you around $200. But it will protect the hardware much better then a milk or vegi plastic case.

Just thoughts, I am NOT a professional electrical engineer.