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
Are you really going to pay 766$ for that mobo? Its waaayyy expensive. Look for regular motherboard with 6pcie and be happy.
I paid 20$ for this in 2020 april last year.