You need a good HBA and SAS expanders. Easiest way would be to buy a 4U server chassis that can hold as many HDDs you can find and feed the backplane from the SAS expanders.
Some of the x470 (ryzen) and z370/390 boards have 8 sata 6 ports... Buy that would make for a weird setup for you and still nowhere near 50. Buy a Nas enclosure and add your own drives
You need a good HBA and SAS expanders. Easiest way would be to buy a 4U server chassis that can hold as many HDDs you can find and feed the backplane from the SAS expanders.