“Thus, the maximum total number of PCIe slots in any PC is 16 (= 65536/4096).”
The limit traces back to the I/O port instructions of the Datapoint 2200. Every PCIe slot pretends to be a PCI bridge, and every bridge grabs 4K of a 64K port space whether a card wants ports or not. QEMU can turn that reservation off for root ports, and doesn’t by default. A 1970s terminal still sets the slot count in a 2023 VM.