Is there a way to find out today how many applications are in the Mac AppStore approximately?
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 Approved: 595452
Total Available Apps: 462144
Total Available iPhone Apps: 411134
Total Available iPad Apps: 132388
Total Available Mac Apps: 6939
I believe there is a Browse button when you're in the App Store in iTunes. As long as you don't do it from within an App Store Category it should give you a complete list of apps. At the top you should see something that tells you that you're in page 1 of X where X is the total number of pages. Multiply that by the number of apps in each page and you have your number. Hope this helps.