The amount of Video Cards a motherboard can support (if using powered risers) is only limited by the number of PCI-E slots, correct? I've found a board (MSI - B250 GAMING M3 ATX LGA1151 Motherboard) that has 2 PCI-E 16x and 4 PCI-E 1x. If I use powered risers there should be no issue right?
Not necessarily correct, though it may be correct in your situation. It appears some people have issues with fully populating the PCI-e slots, though it's unclear whether a BIOS update or reconfiguration would have helped.
I also recently read something on a review site that was testing a motherboard for fun that supported a ridiculous number of PCI-e slots (pretty sure it was more than 10). However, it refused to work when fully populated with AMD cards. All slots were usable when a few nVidia cards were used. Not clear whether this was an OS or hardware limit. Speaking of which, you may also wish to consider OS limits. I seem to recall encountering instances of people running into walls with certain versions of Windows 8 and 5+ GPUs.
I can't say for certain that all motherboards can support as many GPUs as they have PCI-e slots, but I can say that some configurations may involve tofurkey sacrifices and head-banging.
P.S. I'm not endorsing (or discouraging) this particular setup, but you should be aware of things like the potential for fire when dealing with high-power setups and equipment of questionable repute (I'm looking at you, powered risers of unknown origin).
Well I've learned it the hard way.. but apparently, that particular board, MSI B250 Gaming M3 can support only 4 GPUs at the same time, despite there are 4 pci-e slots on it. See the notes under "Slots" section of the specification below: https://www.msi.com/Motherboard/B250-GAMING-M3.html#productSpecification-section
• 2 x PCIe 3.0 x16 slots (supports x16/x4 mode)* • 4 x PCIe 3.0 x1 slots
** The PCI_E4 slot will be unavailable when an M.2 SSD module has been installed in the M.2_2 slot.
** The PCI_E2 slot will be unavailable when an expansion card has been installed in the PCI_E5 slot.
** The PCI_E3 slot will be unavailable when an expansion card has been installed in the PCI_E6 slot.
So the answer is no, it is not only limited by the number of slots, there are also these kind of "not so clear" limitations.