[Hampshire] Network numpty

I am involved in a debate regarding the number of nics needed on a Linux
server.

The Scenario: Two or more PCs need to be connected on a LAN. A PC will be
set up as a server, providing email, firewall, files etc. So all internet
traffic from the other PCs should of course go through the server, connected
up through a hub. A fairly standard set up. Obviously the server would have
to address the IPs of the internal LAN, as well as deal with the external
ISP connection.

The question is - how is all this normally wired up? More specifically, must
the server have two network ports (ie nics)? (One for the LAN via the hub,
and one to connect to the ADSL), or can the ADSL connection AND the LAN be
handled through one network port?

I have never got this involved with setting up a server before, so I don't
know how this is normally set up physically.

Any guidance/abuse/mocking laughter gratefully received...

Kelvin