There are no subscriptions required or needed for the Roku device itself. Some channels might have a subscription fee, such as Netflix or Hulu, and others are completely free, such as YouTube.
You can have as many devices you want on your Roku account. Roku never charges for registering a device or providing support.
But please be aware there are scam sites on the Internet claiming to be Roku support that try to tell you a subscription is required to use a Roku. All they want is your credit card info so they can sell it on the dark web. Some even want to connect to your computer to resolve your "problem", but again they are looking to install something bad on your computer to steal more info.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.