It should be pretty easy to see if you have multiple tabs open. See the attached image, where I have two tabs open to web sites, IMDB and Rotten Tomatoes.
They gave me two instances of the JRWeb.exe process in Windows 7.
However, as I am working on my Workstation, where I have never opened a web page using the internal browser, Norton 360 kicked in, decided JRWeb.exe was doing bad things using heuristics, and removed JRWeb.exe!!! I had to restore it to do my tests.
Is it possible that your Anti-virus is interfering with JRWeb.exe as well, and MC is opening additional copies of it when it retries to open a web page (or you retry by hitting refresh or back and then re-clicking a link)? If your anti-virus is just blocking JRWeb.exe, rather than removing it, that could explain why multiple instances get started.
If I was you, I would reboot my PC and then add in exclusions for JRWeb.exe, JRService.exe, and JRWorker.exe in my virus scanner. ( I assume you already have exclusions for "Media Center 21.exe" and MC21.exe.) Or exclude the whole "C:\Program Files (x86)\J River\Media Center 21" directory. Then test and see if I only got one JRWeb.exe process for each tab I opened to a web page.
Read
this if you have no idea about anti-virus exclusions. It is about Windows Defender, but should give you the idea.