Where are the 12 browsers displayed? Are these desktop icons?

If you mean individual processes in the Windows 10 Task Manager, these are spun up for tab/site isolation as you browse. Check out the internal about:processes page (Shift+Esc) to see which processes are for which content.

It is quite normal to see more Firefox processes running.

• process for the main Firefox thread (user interface)
• one or more content processes
  you see more content processes when Fission is enabled
• process for the compositor thread
• process for extensions
• process for WebRender when this feature is enabled

There are several toolkit.aboutProcesses prefs on the about:config page to show more info. You can find the current multi-process state on the Troubleshooting Information page (about:support).

• "Help -> Troubleshooting Information"
  "Application Basics":<be>Multiprocess Windows and Fission Windows
  "Remote Processes" (Web Content)

You can check the about:memory and about:processes pages for info about all processes. See also the about:unloads page for info about the tabs.

• https://Firefox-source-docs.mozilla.org/browser/tabunloader/