How many Devices can I use on plex

Do I have to pay for each device that used plex
I have 4 devices
Tv, pc, iPad and iPhone

Your TV and PC are always free. Without a Plex Pass, the mobile apps are not free, but a one-time purchase per AppleID. So if you have multiple devices that are connected to the same AppleID, paying for the Plex app once will allow you to use it on all those devices.

Here’s more detail:

To be clear, Plex has no limit on how many devices you can have on your account or use at the same time.