We're only talking 1 gbit ethernet here, so you can have multiple of those ports on PCIe. I have a PCIe card here with two 2.5 gbit on it (don't remember exact

We're only talking 1 gbit ethernet here, so you can have multiple of those ports on PCIe. I have a PCIe card here with two 2.5 gbit on it (don't remember exactly how much it was on Ali but between 20 and 40 EUR) and I can saturate both with iperf3. Since the example only uses 4 ports, it should be easy to make a simple router with just two PCIe cards. But there's probably 4x 1 gbit PCIe out there, too. And if you use 1 gbit fiber, that wouldn't cost much power nor would it need much speed. If your uplink is DSL, you could use a VigorNIC 132.