Finally got around to running a concurrent transcode test on this. So this is an ARC 310 passed through to a Proxmox Ubuntu 22.04 with 6.2 kernel, with hardware tonemapping . I am playing back 6 different 4K UHD rips (remux only) without any errors in playback (skipping etc.).
The native bitrate of the files being transcoded are:
85 Mpbs
64 Mbps
81 Mbps
81 Mbps
42 Mbps
67 Mbps
They were all set to 15 Mbps except the last one. For some reason, on the 6th Plex Web window, when I open the playback options on the overlay, the video quality setting wasn’t there, no matter if I reopened it or resized the window. 
So that one was stuck at 10 Mbps.
I can test more streams if anyone is interested but 6 seemed like way more than I would ever stream since every friend I’ve ever shared my library with only thinks Plex is commercial streaming content, and the only way they ever see my library items is if:
- Go to their house
- Install the Plex app for them
- Pin the libraries I shared with them (which is the only reason they got the invite to Plex)
- Hide internet content, so they don’t accidentally end up in Movies & TV on Plex
- Go into every library and change from Recommended to Library view so it doesn’t say there are no library items
