As can be seen in the datagram, the expansion slots are divided into 3 groups. Each group has its own dedicated 24 Pin ATX12V power supply which makes operations stable in multi-GPU use.
The total power-handling capacity of this connector is 373 watts using standard terminals or 560 watts using HCS terminals. Combining the 24-pin main and the four-pin +12 V power connector results in up to 565 watts (standard terminals) or 824 watts (using HCS terminals) total power available to the motherboard and processor! (from http://www.tomshardware.co.uk/power-supply-specifications-atx-reference,review-32338-8.html)
I have rigs with 6 GTX1060's in that draw less than 600W so, you could easily run this many GPUs with 3 'normal' PSUs.
I'm using 1600W PSUs for most Rigs now - but they're not expensive as I get them from the big Chinese suppliers, or buy chassis which come with them already provided.
If people are going to their local PC suppliers they won't find much over 1000W and will pay high prices, but you can get decent PSUs for mining at decent prices now. Especially for Mobo's like this one where the GPUs are split into groups again.