For decades SHOUTcast had two listener counts, unique and the "normal one" with a simple principe one IP is one person. While this can be true with IPv6 these days it is definitely not the case on IPv4. People often are behind NATs or even carrier-NATs to save the rare IPv4 supply. Many people also hide behind proxies, VPNs and other services. We will still use the IP as a measurement to call a listener "unique" but not as the sole condition.
On every request your browser/media player sends a few things. One of these is an identification of what software is used, in browsers often the OS and browser build is also sent. The chance everybody on one "shared IP" uses the same device, OS and browser (version) is quite small. That is why this will be used as a 2nd criteria to see if a listener is unique or not. Yes you can cheat by opening different programs, but is it worth it?
In a case of for example our benchmark it will all send the same user agent so will be counted as one. Also in some cased the media player may connect multiple times so these will also count as one unique listener.
Updated less than a minute ago