Depends on your budget and use case. Jellyfin’s “Hardware Selection” documentation has you covered as to general specs, so maybe consider them a shopping list.

Briefly, a mini PC with the following will work for a “normal server:”

CPU: Intel Pentium G4560, Intel Core i3-7100 or newer Pentium or better

RAM: 8GB or more

Storage: 60GB SSD storage for Jellyfin files and transcoding cache.

Graphics: Intel HD 6xx (7th gen integrated graphics) or newer, Nvidia GTX 16 / RTX 20 series or newer (excluding GTX 1650). Intel is recommended over Nvidia. AMD and Apple Silicon are not recommended.

If you don’t need transcoding, or can use low power transcoding, Intel 12th gen or newer Atom CPUs with integrated graphics will serve.

https://jellyfin.org/docs/general/administration/hardware-selection/#:~:text=Storage%3A 60GB SSD storage for,Apple Silicon are not recommended.

I’ve run Jellyfin on a R Pi 4 but experienced glitches on playback with a Roku with media that required transcoding. It wasn’t awful, but knowing myself, I knew it’d get old quickly so I went back to MiniDLNA which works a treat when playing back on a mini PC with VLC or Kodi.