Thank you for reply.
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?
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?
One of my rigs is 13 cards, but I'm using the ASUS B250 Mining Export board...
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?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?
On the risers, the only recommendation I would make is replacing the USB cables with quality ones you buy separate - with the risers I've worked with, 99% of the problems I have with them are the cheap USB cables they come with.
Nice