Has anyone tried this?

I wonder how many cards you can stick as much as possible)

yes I have tried that. How well it works depends on the motherboard haswell chipsets cannot go above 7 GPU, skylake (z170) can barely do 5. I think the 2011 socket chipsests can do more but I have not tried. Useless for asrock pro btc motherboards, you can only gain 1 extra gpu.

I have GA-780T-D3L 3.1/4.0= 4 pci-e 1x, 1 pci-e 16x

7 may not be possible with Nvidia either, they use more channels than amd. Unless you get a server level processor like the Xeon, then maybe.