What i don't get is why they don't use 14TB HDDs, they are only 15% more expensive per TB. On the other hand they'd need 2.33x less PCs at $550 each, plus their power use.
So instead of every 7 PCs with 6TB HDDs they'd need 3 with 14TB HDDs.
PS: They could also use a mainboard with 10 SATA ports instead of 8. They are only $15 more than the chosen board. Adding one or more PCIe 8x SATA controller cards might also make sense, depending on the average load of a system.
PCIe lane count depends on CPU support, for AM4 I believe it ranges from 6x PCIe 3.0 (cheapest AMD Athlon/Ryzen CPU-s with integrated GPU) to 24 (PCIe 4.0 for latest Zen 2 based Ryzen 3000 series).
The CPU they list in the article supports 16 lanes of PCIe 3.0 connectivity + 4 lanes for chipset (storage and other IO). Nowhere near the 48 PCIe lanes you mention, although you could argue that 20+4 lanes of PCIe 4.0 bandwidth is equal to 48 lanes of PCIe 3.0 bandwidth, but this would require a compatible CPU, which would increase the cost by hundreds of dollars.
What i don't get is why they don't use 14TB HDDs, they are only 15% more expensive per TB. On the other hand they'd need 2.33x less PCs at $550 each, plus their power use.
So instead of every 7 PCs with 6TB HDDs they'd need 3 with 14TB HDDs.
PS: They could also use a mainboard with 10 SATA ports instead of 8. They are only $15 more than the chosen board. Adding one or more PCIe 8x SATA controller cards might also make sense, depending on the average load of a system.