Currently have 8GB PC3-12800E (2x 4GB) and while it works now, with Plex's transcoder set to /tmp, I often see RAM usage in the 60~70 percentile range. This is only myself transcoding. I can only imaging what would happen if multiple users decide to watch content!
I'm just too cheap to buy another 8GB at the moment. I may just end up buying two 8GB DIMMS and selling the two 4GB DIMMS.
Edit: Typically my server sits around 30~40% useage with only a handful of dockers running
( cadvisor, deluge, DDclient, jackett, sonarr, radarr, plex, plexpy, openvpn-as, plexrequests)
However, when Plex decides it wants to record a show or two at the same time I often see the usage jump to around 80% which doesn't leave for a lot of room to do other things like transcode if someone decides to watch something. (I have plex set to transcode to "/tmp" since I don't want it writing to the disk)