Do I need a separate concrete5 install in each subdomain?

I have just upgraded my site, actually rewrote it, because my hosting company has updated php to V7.3 and my site was way back at C5 v5.6 and php 5.6.4. With only a few pages, I found it easier to just rebuild it with all the new versions. That site is working fine. I also have a site for my wife's books that is currently on another server that no longer works for the same reasons mine didn't. I want to move hers to my account (my public_htmi) as a sub domain that her URL points to. Do I need to upload a separate instance of C5 to that subdomain folder in order to build her site? Or will the instance of C5 in my public_html be able to build her site as well with the site files in her subdomain folder? I guess another way to ask the question is does each site need a unique instance of C5 to build that site?