Select the "Photos" container near the top of the Library Inspector. Set the Browser filter to "Show All".
If you stack your Versions, collapse Stacks. The number of items reported in the status bar of the Browser is the number of Originals in your Library.
Expand all Stacks. The number of items reported in the status bar of the Browser is the number of Versions in your Library.
In addition to Kirby's method, another way to do this is to hold down the "option" key when starting Aperture. In The "Which library...." dialog, the number of total versions along with the number of originals, is listed for the selected library towards the bottom of the dialog.
Neither approach is very convenient. Why they just don't diplay the totals next to "Photos" in the library panel is a mystery.