I mean that's up to you, my personal settings are 2:1, it'll delete and stop seeding once it's uploaded as much as 2 downloads worth.
But it's important to remember that Radarr/Sonarr are Usenet first, torrents didn't have official support till fairly recently and it's still a little wonky
Officially, to seed you basically have to have a duplicate file until your set threshold is reached.
If you want to seed forever (or at least a really long time (ty btw for your contributions)) I'd say you're probably going to want some custom scripts. Have Radarr move the file and rename as normal and then your script to symlink it back to the torrent directory under the original filename so it can continue to seed without taking up double space for every movie