My general rule of thumb is that how ever many 16x slots you have is the most number of GPU's you can put without needing powered risers.
You have 3x 16x slots, therefore you can use 3x GPU's on risers without having to power them. If you add a 4th card but your motherboard only supports 3x 16x PCI slots, you'll need a powered riser.
I run a small farm and have not had a single issue.
Yeah I heard that it's quite small