The unofficial number of apps for Windows 8 from the tracking website, MetroStore Scanner, shows 35,909 Windows 8 apps available.
MetroStore keeps a count of Metro-style apps for Windows 8 and Windows RT and reportedly, approximately 400 apps hit the store each week. WinSuperSite notes:
New app publishing was actually quite a bit stronger in the weeks leading up to Christmas, however. The week of December 10, for example, 700 apps or more were added most days. Over 14,000 new apps were added in the past month.
A look at the contents of Microsoft’s App Store tells us that the education section has the most apps, with 5,560 apps, followed by entertainment with 5386 apps, and then books & reference with 4276. There are 3684 games apps, 3386 tools apps and 2611 news and weather apps. On the ARM platform, 35,645 Metro apps exist, 37,100 run on x64 platforms and 37,028 run on 32-bit x86.
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You’ll notice that the x64 and x86 totals are greater than the total number of apps in the store, but go figure. Your guess is as good as mine. Possibly the former do not reflect pulled apps.
Certainly, not every app is of the highest quality in the Windows Store and Google and Facebook are among the prominent developers who have as yet, not released apps in the Windows Store.
Google in particular, is famously locked in a feud with Microsoft, declaring that the Windows 8 platform is not large enough to merit their app development. Nevertheless, the Microsoft Store is seeing healthy growth with the rate of new apps added.