Example:
In my proxmox server I have 4 nics for network a,b,c&d connected to eth0, eth1, eth2, eth3. Proxmox connects vmbr0, vmbr1, vmbr2, vmbr3. So I have:
a --> eth0 --> vmbr0
b --> eth1 --> vmbr1
c --> eth2 --> vmbr2
d --> eth3 --> vmbr3
Now I have a firewall appliance with 4 virtual nics...