As far as I know, Supermicro makes server boards, not GPUs. The few boards it does make for GPUs are indeed for Teslas but I'm not sure how you will fit 20 of them in a single box.
Also I am only seeing results for Tesla P40 when I type this search term in Google.
What I suggest doing is go through this list
https://en.wikipedia.org/wiki/List_of_Intel_chipsets , and try to find a motherboard with one of the recent Intel chipsets and that also has enough PCIe lanes for powering four (nvidia) GPUs, and then order them in bulk.