How many apps are the Mac AppStore?
I use AppShopper.com to get information like this.  In the right sidebar of every page, it shows the total stats of all the app stores.  For example, as of 18 Sep 2011 at 5:10pm GMT, the sidebar shows:

Total Apps
Total Apps Approved: 595452
Total Available Apps: 462144
Total Available iPhone Apps: 411134
Total Available iPad Apps: 132388
Total Available Mac Apps: 6939