I'm curious; what does tape backup have to do with the number of PCI slots/busses?
I'm not very clear about how tape works in qemu, but the problem is pcie devices under q35. The pcie topology requires one device per bus, therefore the 256 bus might not be enough if we have many pcie devices.
Note that you can have multifunction pcie devices. so you can have up to 8 per pcie slot. Which allows close to 2000 devices. Limitation: They are not individually hotpluggable.