2 or 3 at a time normally but I'm fairly sure it could handle more depending on modpack
If you're doing it you need to get absolutely all the optimisation mods you can for the version and most importantly pregenerate the world. Once you do those things with two or 3 on it'll hover around 80% CPU usage
I will say with 4 players on Infernal Origins it struggles, I suspect because it turns up the mob spawn rate, adds custom mob AI and generates massive unlit caves everywhere for them to spawn in
On the ATM server we have 5 max upgraded apotheosis mob spawner grinders running 24/7 chunk loaded and the server's still pretty snappy still
External video card would probably work well now that you mention it. But at that point, for what I want to use it for, I might as well do an Intel Mini PC since it would use less power.
But good project idea if I ever want to set up a Minecraft server.
Same setup as you, fan and NVMe and I can play 720-1080p video without any issues from all of the streaming platforms I've tried so far as well as local files. Also streaming 1080p 60Hz gameplay from my gaming setup over LAN with no problems.