Microsoft: Windows Store has 669,000 apps for phones, tablets and PCs
Since January of 2014, that would be a 67% rise in the number of apps in the store. Since the figure includes apps for PCs, the actual number available for Windows Phone handsets is less than the 669,000. But as they become universal, the number of apps in the Windows Store will soon represent a more accurate figure representing the number available for Windows Phone handsets.
The amount of apps in the Windows Store for handsets trails the number of apps offered in the Apple App Store (1.5 million) and Google Play Store (1.6 million). But the raw numbers are somewhat irrelevant since some of the most popular iOS and Android apps are not available for Windows Phone. Even Instagram, which does have a listing in the Windows Store, has been in Beta for nearly two years.
It's a Catch 22. Some consumers won't buy a Windows Phone handset due to the lack of some popular apps. Yet, developers won't produce these apps for Windows Phone because of the platform's low market share in many regions. Either demand for Windows Phone handsets pick up, giving developers a reason to write for the platform, or Microsoft makes a shift to Android, like BlackBerry has. And don't forget, Microsoft makes more money each year from licensing its Android patents than it makes from selling Windows Phone handsets.
source: Microsoft via WindowsPowerUser