If you go with what is called "Method One: nsvhr/nsunix" in the Howto, then there is no need for the slave servers to listen to any port at all - they will be connected to the master via a unix socket. Every user request goes through port 80 of the master server and therefore every user will only ever see port 80 on the master.
It's also possible to connect master and slaves via nssock - this is called "Method Two: nsvhr/nssock" in Jerry's Howto. In that case the slaves listen to some ports like 8001. The master server only needs to listen to external requests on port 80, and redirects the requests to the clients via the socket, e.g. 8001.