found a 19 gpu motherboard on ebay :
https://ebay.us/m5SlB0it have 19 slot PCI
3 power supply inputs
does it support PCIE gen 3
You can use google? Or maybe look at the asus page? Manual? It needs the same time to type your text

And yes it has PCIe 3.0 and it doesn´t matter, for mining you can use pcie 1.0 it makes no difference in speed