Как то так было, может в 6 поменялось что
The main reason of fixing the number of devices, is that now proxmox use fixed pci slot for devices, so we have 31 pci slots available.(X vitio-disk, X vitio_net,ide,videocard,scsi controllers,usb controller,....)
with proxmox 1.X, pci slots was not fixed, so sometimes this cause bug in windows (network address change by example),disk order bug, and some other bugs.