Well, one specific thing I was thinking of was the limit of 15 USB ports in El Capitan and later. There's no reason for that to exist in an absolute sense, but no real Macs have enough ports to run into trouble.

Doesn't the Mac Pro get real close though?

2 built-in USB C ports on the top, the I/O card has 2x USB-C and 2x USB 3, and each GPU has 4x USB-C.

So with dual GPUs that's 14 USB ports. Maybe some are implemented via internal hubs?

PCIe cards don't count. So ignore the ones on the GPUs, I/O card, etc.

If not for that, it would in fact go way over, because USB3 ports count twice.

Do the cards actually have USB controllers on them though? I thought all the USB-C ports on the Mac Pro were routed through the motherboard in order to support using any port for displays irrespective of what GPU is driving it. Or is this one of those weird Thunderbolt vs USB things?

Oh—I have no idea then, sorry! Actually, as far as I know, Apple could have fixed the port limit bug in Catalina, since I've never set up a Hackintosh on that OS. Kind of hoping a proper Hackintosh developer will chime in here because I'm not really qualified!

What about USB hubs? Are you limited to 15 USB ports total?

No, only ports on the motherboard. I think the limit is technically per-controller, but I'm not sure and I don't want to say something wrong. If you add ports via a PCIe card, those don't count against the limit either.

That said, the limit is more problematic than it initially appears, because USB 3 ports count twice—once for USB 2 devices, and once for USB 3 devices. Some motherboards also use USB under the hood for things like Bluetooth (as do real Macs, btw), and even USB headers which aren't connected to anything will take up space if you don't explicitly exclude them.