I think it was a DIY machine, those RTX 3090 have gotten cheaper for sure. From my experience, going beyond 4 GPUs is a pricey affair. See [§]. All but one model of the RTX3090 require at least 3 slots.

If 4 GPUs connected via PCIe 4.0x16 are enough you can choose among various sRTX4 boards for 3000 series AMD Threadripper CPUs.

[§] https://www.reddit.com/r/deeplearning/comments/tw0olq/commen...

Another useful URL: https://www.pugetsystems.com/labs/articles/Quad-GeForce-RTX-...