Do as many copies as the number of revisions exist for a file in plone?
If you change the content in any way (or just re-save it) a duplicate of the object is created (which allows you to undo later).  If you change only the metadata (like the title) the object is usually not duplicated.
These duplicated "backup" copies are removed (and the undo option for them) whenever the database is packed.
There rules dependent on the object being persistent: that's almost all normal Zope (and Plone) objects.  Some exceptions may exist, but they are rare.
In plone, how many physical copies of a file (or any content) exist if it is revised say 4 times? I am using plone 4.1 wherein the files and images are stored on the file system.