You don't need an account on every service.
One account will usually let you interact on other services. From your point of view it will look like you're just on Mastodon, but you're actually on other services too.
For example, if you want to comment on a PeerTube video, you can do that from Mastodon. From your point of view it will look like a Mastodon thread, but when you go to the PeerTube video's own page the comment you made on Mastodon will be there.