Looking more at the motherboard, you might get 1 GPU on the motherboard and with powered risers 3 more GPUs.  However, I don't know if standard PCI slots on the right will work with the 750ti to get you 6 cards. I would recommend the H81 Pro as it is designed for mining.  https://www.asrock.com/mb/Intel/H81%20Pro%20BTC%20R2.0/index.asp 

If you do get to 6 cards, you would need more like a minimum 650Watt power supply.   Each card draws less than 40Watts however you need to make sure the PSU is running at about 80% of peak.  Also you need overhead because the motherboard will use some power as well over the 40Watts.  If using powered risers you would need to have enough power connections to connect the powered risers.  Max of 2 GPUs per cable from the PSU. One per is preferred but you most likely would need a larger PSU for that many connections. 

When I was running 6 750 ti on a motherboard it was using 453Watts from the wall according to the Corsair Link and at 94% efficiency.