There’s people running old raspberry pis with USB hard drives.

It’ll run on just about anything.

Though, you’ll only be able to stream original quality, no on the fly quality changes for low speed connections and such.

NUCs (specifically Intel 8th?-gen or later) are pretty much ideal for serving Jellyfin because the Intel integrated graphics can do video transcoding and the software is actually not very demanding otherwise, so the low-power CPUs are fine.

If you were buying hardware specifically for Jellyfin (i.e. didn’t want to cobble together something used), I’d suggest an N100 or N150-based NAS mini-PC like this: https://www.bee-link.com/products/beelink-me-mini-n150

Running on a 2016 intel processor with an old Nvidia 2070. I can watch about 3 streams on different devices at the same time.

You absolutely can. Currently running mine off of my desktop, but it’s overkill and have a few coworkers that used nucs/cheap mini PCs to setup jellyfin. Biggest thing is to make sure you have enough storage to hold all of your media, then you’re fine.

But then won’t your rack look a bit empty?

I could do something simple like a NUC or equivalent and a tiny NAS

Literally me rn. A tiny second-hand Dell Optiplex with a 8th gen i3 and a 2TB SSD.
Maybe not the most future thinking solution since it can only fit 2.5" HDDs and NVMe’s, which are both more expensive than conventional big fat HDDs, but hey, works great.
If you can find one for cheap locally and get a decent deal on a compatible drive you’re set. You could stretch a Terabyte or two for a while as long as you’re not trying to host Jellyfin for too many people (and are OK deleting watched Shows/Movies when you start running low)

Your biggest issue self hosting JF will likely be port forwarding and the infrastructure required so external users can connect via WAN. Plex abstracts a lot of this away since you just have to open the port and Plex will access your library and broker the data between your server and the client. This now requires the server owner have a Plex Pass. The easiest way to host JF is allowing users LAN access via WireGuard (pivpn) but then you start hitting limitations to which devices they can install WireGuard on (smart tvs). I use Plex and JF, Plex for external users since I bought the lifetime pass 10 years ago, and JF as a backup as the enshittification of Plex continues full steam.

You can definitely run it on those sorts of machines. You’ll want plenty of storage, but apart from thst it’s not too demanding if you don’t load it up with very high res videos.

i mean i’m only using it for me and my family but i just run it on my desktop linux (and before i switched i was running it on my desktop windows with scoop install jellyfin)

I just run it on my gaming tower in a container

Your biggest issue self hosting JF will likely be port forwarding and the infrastructure required so external users can connect via WAN. Plex abstracts a lot of this away since you just have to open the port and Plex will access your library and broker the data between your server and the client. This now requires the server owner have a Plex Pass. The easiest way to host JF is allowing users LAN access via WireGuard (pivpn) but then you start hitting limitations to which devices they can install WireGuard on (smart tvs). I use Plex and JF, Plex for external users since I bought the lifetime pass 10 years ago, and JF as a backup as the enshittification of Plex continues full steam.

an entire data center

What are you on about?