A project is stored in a file, so technically you can put it where ever you want it.
However, you should consider using shared resources to enable you to share resources across multiple projects.
Then you can avoid overbooking resources even when projects shall run in parallel.
To be able to do this, Plan needs the resources to be created in a separate file, and the projects must all reside in the same directory,
If you run all projects yourself, you could just store everything in your home directory, e.g:
~/Plan !- SharedResources.plan !- Projects !- <project1>.plan !- <project2>.plan !- etc
If there are multiple project managers running different projects that need to share the same resources, a similar structure should be in a place where all managers have access.
All managers must have read access to the shared resources file, as well as read access to all the projects.