What is needed to go from single Ethernet to many?

If this is for a prototype - consider adding a USB ethernet adapter.

If you're building a product, I'd consider an onboard ethernet switch chip. Like this:

http://www.micrel.com/page.do?page=product-info/fastether_sw.jsp

_{(source: micrel.com)}

The ADM6996 may also be an option, if you can find it.

If your processor only has one PHY, you're not going to get a lot more speed by embedding the switch on your board. It will be much easier to just add a switch:

As the text on this example indicates, any halfway decent switch will autonegotiate 10/100/1000 Mbps, cable crossover.

Switches are available in anywhere from 4 to 48 ports, with 5 ports being a common number for simple desktop switches. They can sit on your desk or be rack-mounted. You can chain them together for ridiculous numbers of ports, if you feel so compelled. You can get them for less than $10 (visit Newegg), or as much as 10,000. One important thing to consider is whether you want a managed or unmanaged switch, but we'd need more information to make this decision, and this is swiftly turning into a question for ServerFault.

An external switch is probably the most flexible, simple, and cheap way to do what you want. It will be physically larger than putting the contents of a switch on your PCB, but I wouldn't do that unless it was absolutely necessary.

If each port is going on a completely separate network you can get away with a single MAC address, but this really doesn't work if you are going to have it on the same network.

I would highly recommend having separate drivers with separate MAC address per port.

My answer is focused on if you were creating a device like an embedded firewall where you were wanting traffic to come in on one port, filtered, and then be sent back out on a different port going to a different network.