server is only beeing used for family members, so its seldom more than one person watching at the same time (at max two persons).
watching both 4K and 1080p, but always direct play/stream, no transcoding (other than perhaps the audio sometimes since using sonos on one TV).
Not open to play “remote” at all, only “in-home” watching. and if i in the future would use it i wouldnt transcode 4K (would download a 1080p instead and transcode that if i couldnt direct stream it)
I’d start with 4, then go up or down as you feel out performance.
No one is going to have an exact number for your particular setup, you will only figure out by trial and error.
Keep in mind, some tasks are still single threaded, so core count is not the only performance consideration, how fast (ie cpumark) a core is can matter a lot, especially if HD audio + subtitles are transcoding (single threaded).