Did you order and the rig is up and running?

Do you have all psu connected to the motherboard? You were talking about 4 psu, but I thought the mb only needed 3?
Can the gpu draw power from the board or do you need powered risers? On the website it seems like enough to connect psu to mb.
Risers gets power from the psu AND gpu gets power from psu?

I would recommend AGAINST those Gigabyte cards - they really went cheap on their fans on everything BUT the Aorus line the last couple-three years.
They work well enough as long as the fans are running though, and decent cooling, but I've had fans go out on *2* of them (1 fan each) in the last month, and the fans are deliberately designed to be a nightmare to try to re-lube (not that re-lubing lasts long in ANY case).

EVGA, MSI are all ball bearing fans (except PERHAPS the MSI blower models, I've not gotten confirmation there either way) - I'm fond of the EVGA SC "black" models for their consistantly good cooling, dual ball bearing fans, low level of fancy "pretty" LED lighting that just wastes money on the card design and construction, and usually low pricing.

The graphics cards won't be drawing from the PCIe Riser as long as you have the PCI-E connected directly to the video card.  My rigs run just fine on SATA power connectors directly to the Riser.
6 GTX 1070 + MB + CPU + RAM + SSD = 75 watts idle and 860 watts at full hashing load (cards set to 90% power).  I run that 860 load on a single 1000 watt PS

Yes, you only need a single sata for your SSD (64gb MFC is plenty) if you choose to use an SSD, some just boot off usb flash
The two 850s will each have about 8-12 SATA connectors, plenty to power the risers
CPU/MB/SSD/Risers will use 100-150 watts total

Yes I'm using USD

Even the spread, you would run 5 less riser cards and the 675 Watts is going to save you >$2.50CAD/day ($912/year)

Plus you will save on the cost of the power supplies, not needing as large of wattage nor will you need 2 circuits.  The two 850s will run on a single 20 amp safely

ROG -STRIX-RX570-4G-GAMING  13 x $280.39 = $3,651
Hash Rate NeoScrypt is about 8,000 kH/s
TDP 150W (90% is 135W)

Asus GTX 1070  8 x $433 = $3,464
Hash Rate NeoScrypt is about 8,000 kH/s
TDP 150W (90% is 135W)

5 LESS cards will result in the same hashing power for $190 LESS!

You will also save 675 watts and have more room to add on later!

That 1800W power supply is NOT rated for 120V US power.....  It might work, might not!  You need about 1300 watts at 100% or 1180 watts at 90% card power.  
Two of these will suffice for $120/ea - https://www.newegg.com/Product/Product.aspx?Item=N82E16817151188

G3930 is cheaper - https://www.newegg.com/Product/Product.aspx?Item=N82E16819117748

For the power supplies, I'd recommend getting 3 of these:
https://www.aliexpress.com/item/New-and-original-Gold-POWER-1800W-BTC-power-supply-for-R9-380-RX-470-RX480-6/32825827647.html?spm=a2g0s.9042311.0.0.xNt9Ym
Im running a mining farm with 48 of these PSUs for the past 8 months, and none of them have failed, you get ample power, and running 6 cards on each.

 You CAN run them safely via MOLEX adapters - and there are some with all 3 connections that are the recent "4 cap" versions.
 The COBOC ones Newegg sells as an example, seen the same design from a few other places.

 You don't HAVE to run 19 cards from the ASUS B250 Mining Pro to make it worthwhile - the on-board power supply management is nice, and saves a few $$ + is simpler vs having to use Add2PS type adapters.
 If it's lower cost vs a 13-slot MB, it's DEFINITELY the better choice.

 NewEgg has a Canadian branch, can't hurt to check them in addition to the other sites you mention.

 I strongly recommend use of the EVGA G2 supply line over the G1 or the GQ or the G3 - the BALL BEARING fan is going to outlast the "fancy name sleeve bearing" junk in the other lines.
 I'm not 100% sure on the G2L line - I think it's also ball bearing fan, but the power connections are kinda wierd and do NOT match up with anything else at all.
 I can also recommend the Seasonic X-series, but those are in very short supply and getting way expensive when you CAN find them any more.

The problem is not Windows. The problem is the Asus B250 mining expert REQUIRES using Nvidia P106 mining cards with more than 13 cards on the board. BBT was able to run 21 cards, 13 AMD RX 570 and 8 Nvidia P106 in Windows 10 with the latest Crimson Relive drivers on the Asus B250 mining expert. I don't think anyone has been able to run more than 13 AMD cards in Linux on this board either.

For the power supplies, I'd recommend getting 3 of these:

1x 570 consumes around 100W from the wall mining ethereum solo, and 140W from the wall dual mining.
Even thought those are average numbers, in any electrical components, there will almost always be that power surge that occurs for a split second; you do not want your PSU to blow up because of that. Thats why I'd always recommend anyone to calculate their power requirements to be at 80% of the PSUs capacity; so if you have a 1000W PSU, try to calculate that your components connected to the PSU to run at 800W, and leave that extra room for electrical surges.

18 cards will not run on Windows as of now, we are expecting an update in december that will allow windows to recognize 18 cards; where currently linux is the only way to go for more than 12 GPU. (Simplemining OS is a nice noob friendly linux OS for miners)

With the asus motherboard, you do not need the dual/triple PSU cables. You may hook 3 PSUs to the motherboard, and it does the job on its own.

The graphic cards are too expensive, even with canada's inflated prices.
A quick check on newegg.ca , I was able to build a cart of 18 RX580s, but different vendors, that can easily save you 500$ and If I were you, I'd configure each card's overclock/undervolt through their bios (time consuming, but worth the effort).
Prices are inflated as well in my country's local market (Dubai, UAE), but what I've done is I found the distributor's for these graphic cards in my country, and Im purchasing directly from them. In my case it was sapphire.

I would recommend getting a bigger harddrive, or at least a second cheap non-SSD drive for virtual memory. I had this issue before when I upgraded my rigs from 6gpus each to 12 gpu where claymore's cryptonote miner (cryptonight) requires to have at least the same amount of VRAM as virtual memory. In your case, it would be 18x 4GB which is 72GB + your OS which is around 20GB; a 120gb SSD would be a better option, and it isnt that much more expensive.

Get a server power supply plus breakout board and a couple 6pin male to 8pin male cables in addition to a desktop psu.  I have some hp's that will do 900w at 110v.  That's the cheapest way to power it. 

You don't want to run PSU's close to the rated the capacity. They won't be as efficient and it will shorten it's lifespan. 50% to 80% of the rated capacity is where PSU will run most efficiently and cool. The other consideration is how many PCI-E connecors you need for all the GPU's and risers. Many PSU's less than 1000W will only have 4 PCI-E connectors per PSU. For 13  RX 570's you will need a minmum of 13 individual 8-pin connectors just for the VGA power. Some cards like the Nitro+ also have a 6-pin connector that may be optional. For risers I use 6-pin PCI-E risers only and connect two or three risers per PSU 8-pin cable using a PCI-E 8-pin to dual 6+2 splitter cable, figure another 5 PCI-E PSU ports for the risers, so in total you need a minimum of 18 PCI-E ports.

Mostly RX 480/580 8GB cards. Asus STRIX, Sapphire Nitro+ and  MSI Gaming X. Also a Saphire Pulse RX 580 4 GB and a Nitro+ RX 570 4GB with a EVGA 1600 W T2 for the motherboard, all 13 risers and 4 cards VGA power on a 120V 20A outlet together with a EVGA P2 1000 W + G2 850 W for the rest of the cards on a 15A circuit. All the cards have a Polaris Bios Editor v1.62 'one click timing patch' Bios mod and I get the best result with the Sapphire Nitro+ RX 580's 30-30.5 MH/s ETH 875 MH/s DCR dual mining.

I wouldn't hold my breath waiting for Asus to release a firmware update. Dual mining ETH + DCR a RX 570 will pull ~140 W between the GPU and riser and a RX 580 ~180 W. I have a 13 card mixed card build on a H110 Pro BTC+ dual mining ETH + DCR and it's been rock solid. Total power draw at the wall is 2475 W with a triple ATX PSU setup.

I'll go with Vann's advice, even you manage all cards to be working on one Rig this is not practical for since it has a centralized failure, mining is not a 100% reliable all the time of the day, if any of the GPU's, PC components, Softwares or even connectivity failed, your profits will greatly reduced.

Also, each RX470 will draw 180-190w  in dual mining, 4x650w PSU in 18GPU is not sufficient to power them all

You won't be able to run all 18 cards on the same motherboard. Running more than 13 GPU's with the Asus B250 mining expert REQUIRES using Nvidia P106 mining cards.

https://bitcointalk.org/index.php?topic=2357461.msg24031325#msg24031325

If you want to run all AMD cards I would suggest breaking it up in to a 12 card build on a ASRock H110 Pro BTC+ and a 6 card build on a Biostar TB85 or another H110 Pro BTC+ motherboard if you plan to expand.