You can run 1 web server or 2. You need at least one listening on :80; it can either pass the request off to a module (like mod_php, mod_perl, CGI) or it can pass the request off to another web server. The 12factor site is espousing this second approach, separating the front-end server (perhaps nginx on :80) from the application server (perhaps apache on :8080/3000, running mod_whatever, or in their case, a standalone app server like Jetty/Tornado). Almost any web server can run as a reverse proxy, but mod_php/mod_perl usually require a specific server like Apache.

If you've got a dollar, and an hour, maybe run through setting it all up [1], then run lsof -i at the end to understand what's listening on what ports.

[1] https://www.digitalocean.com/community/tutorials/how-to-conf...