8GB should be more than enough. Plex recommends 2 GB, and user reports seem to agree.
https://support.plex.tv/articles/200375666-plex-media-server-requirements/
https://www.reddit.com/r/PleX/comments/2s4ifc/how_much_ram_do_you_have_on_your_plex_server/
If you're planning on having up to 10 simultaneous streams, I suspect the bottleneck will be your HDD (can't keep up with reading multiple movie files simultaneously) or the CPU (if transcoding). Not RAM.