One login only applies for RDP (Remote Desktop Protocol) session.
You can create as many users as you want on one machine. Login locally with different accounts with different sessions and switch between them. You can login remotely with only one user, however.
Windows client OS (XP to W11) should be up to 10 connections for network sharing (Windows login) if past rules still apply (since XP)
http video streams shouldn't have any limitation in it's tech since it has nothing to do with Windows login. But you have to check Plex license term.
You don't need a server. Windows 11 Pro is fine. You also don't need the Workstation version.