External enclosure connected via usb-c. It's cheap and effective.

It depends on the PC. If it's a mini ITX board with an unused 16 lane PCIe slot, you can put an adapter in there with 4 NVMe drives. Make sure the motherboard supports PCIe bifurcation though.

Another option is an M.2 to SATA adapter. They will connect 4 to 6 drives to an M.2 slot. Finding a place to mount those drives could be tricky though.

Well, what interfaces does it have?

How much space does it have inside?

How much power can it supply on the 12v bus?

Most likely the answer is "very little" or "not at all".

I have a SFF box that can support, max, 3 SATA drives (2.5"), because of lack of ports and little space. But it has the power to run 5. So I put in a PCI SATA controller, and wedged them in. Runs fine.

There are USB enclosures that provide more than 1 drive; what you're after is a 'usb attached das'/'usb direct attached storage'.

Caveat with these is not all of them are the same, and you'll want to validate the chipset they use works for your use case/OS or you can end up with a lovely pile of drives, all with corrupt data. (Cheap is not your friend here.)