something that might be useful to you for that; you can get m.2 slot network adapters which you could use on (some) ITX boards. there's at least 1 & 10gbe ones available.
inb4: CPUs as well as commonly used chipsets have enough lanes, it's not that.
Either way, if you're using an APU then that frees up an x16 PCIe slot, which on many motherboards can be adapted into 4 M.2 slots. That should get you plenty of networking.
What's your goal here??? Multiple NIC in ITX form factor, for what?