I can confirm that this two motherboards with 6 PCI-e and 1 m.2 sockets work stable with 7 GPUs.

The setups are the following:

Motherboards: MSI Z270-A PRO and MSI Z270 Gaming Plus
CPU: Intel G3930 2.9Ghz
RAM: DDR3 2133Mhz 4GB
PSU: XFX 750W x2
GPU: MSI Gaming 1060 3GB and 1070 8GB
Riser cables ver 006S / 007S
m.2 to PCI-e adapter

OS: Custom Linux and ethos 1.2.3

BIOS Settings:
PCI Subsystem Settings: PEG0 – Max Link Speed = Gen2
PCI Subsystem Settings: PCI Latency Timer = 96 PCI Bus Clocks
PCI Subsystem Settings: Above 4G Decoding = Enable
Integrated Peripherals: HD Audio Controler = Disabled
Power Management Setup: Restore after AC Power Loss = Power On
Windows OS Configuration: Windows 7 Installation = Disabled

I need to test these same settings with AMD cards but it should also work.