That would depend on how you are planning to use the Plex Server. I am running mine on an i5-3570K. I can record up to 6 shows at the same time with no issues. All of the clients connecting to my server both local and remote are all capable of Direct Playing all of the content I have recorded and transcoded from the mpeg2 TV video stream to HEVC. The only client I tend to "test" with when needed is a Roku Stick which requires my server to transcode everything for it since it can't handle HEVC or the Live TV mpeg2 video. In my home I use on a daily basis a Nvidia Shield 2017 16GB in our Den, an Amazon Fire TV in Living Room which will soon be replaced with another Nvidia Shield 2017 16GB, and in my home office an Amazon Fire TV Stick. My son uses an Amazon Fire TV Stick in his apartment and a friend uses either a Chromecast Ultra or now she has purchased an Amazon Fire TV Stick as well. I never notice any trancoding happening on my server when users are viewing recordings or in my home watching Live TV.
If the client devices you will be using to view content can't handle the format of the video then you will see transcoding on the server and you may soon want a new server. However, if you choose clients wisely and they can Direct Play your content more than likely your server won't hardly notice multiple streams happening at the same time.
I try to have everything Direct Play whenever possible. Currently we have Xbox One(s), Roku Express Stick, 4k FireTV, Fire Stick, LG Smart TV, Samsung Smart TV(s), & Plex Media Player on a couple computers. I haven't set up any live TV recording yet, but I won't set that up until later this year or possibly early next year.