Here is the lane breakdown on E377:
 

 
Given that there are only 16 PCIe lanes on the board, something's gotta give.  In this case, it's the SATA ports.  The M.2 Key-E slot shares with PE3 (an x1 slot), but I'm going to assume you won't be using that. 
 
PCIe x1 - All lanes can be filled without affecting other slots (with the exception of PE3 - shares with the M.2 Key-E slot).
PCIe x16 - The x16 slots (PE2 and PE4) only share with each other, so using both would drop the bandwidth to x8/x8.
PCIe x16 (electrical x4) - PE6 pulls straight from the PCH, and doesn't share with anything else.
 
M.2 Key-M (top) - Shares with the top 2 SATA ports.
M.2 Key-M (bottom) - Shares with the bottom 2 SATA ports.
 
If you use both M.2 Key-M slots, you will only have the middle SATA ports left to use. 
 
I can't attest to the viability of M.2 to PCIe adapters.  We don't test with them or try them out.  I don't see an issue with trying for 6 GPUs via the PCIe slots, but I strongly suggest you plug in the optional 6pin PCIe power connector at the bottom of the motherboard.
 

I have the z370 FTW and have been able to run 6 GPUs on the PCIe slots but have been unable to this point to get the M.2 to PCIe adapters to work with the PE2 and/or PE4 PCIe slots utilized.
 
The M.2 adapter works as long as I don't populate either PE2 or PE4 slots.  If I do use either of those and the M.2 it gives me a boot error as it is looking for the boot drive to be in the M.2.  It also won't let me enter the bios and hangs on Q code A9.
 
If I populate all PCIe slots and the M.2 the board wont boot and hangs on Q code 96.
 
James

 

Maybe For the use of Mining Only, but it looks like that is what the OP is looking for.
What Risers did you use?
Post a image of your setup!




Yes this is for mining.  Not sure how to solve this as I've tried many different combs of settings.  All risers and cards are verified to be working.  I have verified that the at a minimum the white M2 adapter is working.  I haven't got both M2 adapters working at the same time yet.
 
As requested pictures are below.
 
 

All cards work in the current configuration just not in the M.2 adapter with PE2 and PE4 filled.  I'm pretty confident the board will do it, I just haven't found the right combination of settings.
 
 

I have tried gen 1 and gen 2. Both seem to have the same effect.

Some people have mentioned +4g encoding is needed to be enabled on some brands of mobo for multiple gpus. I haven’t seen that setting on the EVGA boards I have.

nothi
Another question...
 
My last motherboard, even with the two M.2 slots filled up using NVMe, I could still use 4 of the 6 SATA ports.  With this MB, it looks like with the two Key M slots filled using NVMe, I can only use 2 of the 6?  Has anyone tried yet?  How can a MB that cost half as much as much as the FTW be able to do this but the FTW can't, is there something I'm missing?
 
Edit:  The more research I do, the more frustrated I get.  Some boards even let you have more ports available depending on whether the m.2 device is SATA or PCIe, why is the lane sharing the way it is on this board?  The Micro ATX does it better.  The classified K does it better.  4 ports lost for 2 m.2 devices seems ridiculous. 

I was able to get 4 PCIe slots working with the M.2 slots filled with pcie adapters, but not the main 2 16x slots.  I haven't tried it with actual NVMe drives installed.  It looks like it should work though.  i don't have a NVMe drive to test with it though.  

nothi

abbottjamesr
 
I was able to get 4 PCIe slots working with the M.2 slots filled with pcie adapters, but not the main 2 16x slots.  I haven't tried it with actual NVMe drives installed.  It looks like it should work though.  i don't have a NVMe drive to test with it though.  

How were you able to do that?  There is a hard setting in the BIOS that automatically disables sata ports 1/2 and 5/6 when you enable M.2 .

I wasn't using any of the sata ports.  Just the 4 small PCIe slots and the two M.2 slots with PCIe adapters.  When I tried the 5th pcie slot with both M.2 occupied it wouldn't let me into the bios and stated there was no boot drive.

rgrosvenor007
So I ended up buying the Z370 Classified K.  I am able to get 6 GPUs working using all available PCIe slots.  I can get the M2 to PCIe adapters to work but in combination with the 6 PCIe slots being used.  I am trying hard to get 7 GPUs to run on this motherboard and can't do it yet.  I have tried a 1 to 4 PCIe adapter and it works but still can't get all 7 cards to install.  I was able to get 7 cards to Boot but then always have one GPU that says there are no available resources and need to disable something.  I have everything disabled and won't work.  Any ideas?  I tried disabling the audio functionality of the cards and that did not help free up resources.  Please help.  Thanks.

Personally, I think that we wont get more than 6 GPU's to work unless EVGA releases a BIOS update that has "above 4g encoding".
 
Its too bad cause it seems like a nice board.  I will likely sell mine and get something else.
 
James

ASIAN TSUNAMI
James,
How are you getting 6 gpus to mine. I cant get past 2 gpus. When I install the third card, the whole thing just glitches and I get the blue screen of death and sometimes i cant even get into windows. Will you please help me? i've been struggling with this for 2 weeks now.

Assuming Windows
Put one card in PCIe 16x slot
Goto Bios
Set PCIe to Gen 2
Set boot to UFEI mode
Install Windows
- keep network cable unpluged till through all the region stuff.  only plug it in when you get to the network part.
- load all drivers
Turn off PC
Install one or two more cards and start up into windows.  Windows should find them and start installing drivers.
If successful shut off and install remaining cards in all PCIe slots.
 
If that doesn't work let me know and I will go into the bios and send pics of my settings.
 
If you are on linux, make sure you are on Gen2 for pcie and boot it set to legacy.  I didn't have any issues running 6 cards on SMOS.

So I'm in Bios, click on "memory tab", on the left side, under "memory information" it says "XMP #1 and XMP#2.  Only XMP #1 shows information and XMP  #2 shows no information.  XMP#2 is blank.  Is that normal?

In the end, it was the RAM.  I swapped them out with known good RAMs and the motherboard and everything else worked out.  I have 6 gpus running and stable for the last 10 hrs.  I haven't tried the M.2 and i'm not going to install adapters to it.  Thank you for all your help.