The live channel don’t come from Plex. Plex is only relaying them from the tuner to your Plex clients.
Therefore you need a tuner device (or several of them) which can provide the number of parallel streams you need in your household. There are models with 1, 2 , 4 or even 8 tuners on the market.
The HDhomeruns usually have 2 or 4 tuners. Take a look into the technical data.
Keep in mind that you also need an extra tuner if you want to record a channel while you watch a different channel.
You should check first, which channels are available OTA in your area. (And which are unprotected/unencrypted). Plex won’t stream protected channels.
https://support.plex.tv/hc/en-us/articles/115007689648-Watching-Live-TV
