I recently found a motherboard that accepts up to 8 graphics cards (and therefore GPUs) directly, without the need for riser cards etc.
It is marketed as a Mining computer for mining Bitcoin etc., but it struck me that if you could plug in say 8 GTX1080s (or whatever used Nvidia cards I found on eBay), then it could also be a pretty meaty renderfarm.

Does anyone know whether Blender would access and fully use all those GPUs if it was run on such a system?

The motherboard in question is marketed as "Biostar TB250-BTC D+ LGA1151 SODIMM DDR4 2400 8 GPU Support Mining Motherboard" and is not very expensive.  Other, similar motherboards are available.  The actual board isn't important, it's whether Blender would fully utilise the GPUs.

I currently use Blender 2.79, but am about to move up to 2.80.
[Edit] Now using 2.81.