As long as you have one homeplug connected to your router, you just need 1 for each PC you want to connect, and you can have as many as you like iirc.
Worth noting you can also use homeplugs from different manufacturers, so long as they're the same speeds they will all work together (i.e. don't try and mix 200mbs and 85mbs).
Best invention of recent times IMHO, couldn't believe how well they worked and how little faff was involved when I first got my pair of Devolo. Have added a Solwise (from OCUK) since for my sons 360 and have never had a glitch.