WTF!!! You gave me a super-awesome info! 
I wanna kiss you lol 
A consumer-grade motherboard with 19 GPU slots by Asus! I immediately changed my mind. I'll go for it. Does it run smoothly? I mean is it plug and play or do you do need to play with settings in order to make it work with an high numer of GPUs?
I wanna kiss you lol A consumer-grade motherboard with 19 GPU slots by Asus! I immediately changed my mind. I'll go for it. Does it run smoothly? I mean is it plug and play or do you do need to play with settings in order to make it work with an high numer of GPUs?It's pretty much plug and play - as the name if the board implies, this is made for mining. I always upgrade to the latest BIOS, but beyond that it's ready to rock. It actually does one thing that's super helpful - when it boots up, it shows the motherboard and all the slots, and makes them green if they're good or red if not. You can then super-easily tell which riser is giving you grief.
Now I'm gonna recalculate all things since I want to make a system that allows me to add 1 PSU (same brand, same wattage) every 6 GPUs (same model). Probably the AX1500i (1500 Watts) is too much but its smaller brother the AX1200i (1200 Watts) is probably too close to the limit... I mean...
1st array of 6 GPU (max 150 watt each) --> 900 watt
2nd array of 6 GPU (max 150 watt each) --> 900 watt
3th array of 6 GPU (max 150 watt each) --> 900 watt
But one of the PSU should also power the motherboard, CPU, RAM, SSD etc. so probably the best option is...
1st array of 6 GPU + System --> ≃1200 watt powered by AX1500i
2nd array of 6 GPU (max 150 watt each) --> 900 watt powered by AX1200i
3th array of 6 GPU (max 150 watt each) --> 900 watt powered by AX1200i
Is this a good idea?
1st array of 6 GPU (max 150 watt each) --> 900 watt
2nd array of 6 GPU (max 150 watt each) --> 900 watt
3th array of 6 GPU (max 150 watt each) --> 900 watt
But one of the PSU should also power the motherboard, CPU, RAM, SSD etc. so probably the best option is...
1st array of 6 GPU + System --> ≃1200 watt powered by AX1500i
2nd array of 6 GPU (max 150 watt each) --> 900 watt powered by AX1200i
3th array of 6 GPU (max 150 watt each) --> 900 watt powered by AX1200i
Is this a good idea?
Whenever possible I just use one PSU - usually a server one like the DPS-2000BB w/ a breakout. It's by far the cheapest option, and will give you great quality power. With 13 1070's, you'll be at the limit of a DPS-2000BB, so get the DPS-2500BB, which has 2320W. On my 13 card rig I'm actually running 2x 1650w PSU's.
Good luck with the build - it's a great motherboard, and a beast of a machine.