You can use it on as many devices as you want, as long as they have the same IP address. The service will not work if you log in from 2 separate IPs, and you could get banned for that. As long as someone isn’t streaming from your home IP, you can log in while visiting your parents to watch. Basically just make sure if you use multiple devices they are on the same Internet connection.
If you go to the help section and search for “IP” you can get more info!
Image for people who don’t want to log in.
Kodi + Real-Debrid. Or like top comment says you could also use Stremio + Real-Debrid. I recommend using a Trakt account too to keep track of everything you watch/episode progress. Real-Debrid costs about $3 a month but has just about every torrent cached to their servers (it’s never needed to buffer for me), and if they are missing one you can cache it yourself. Setting up kodi/stremio with RD is definitely worth a Google!