According to Metrostorescanner, a 3rd party site tracking Windows Store apps, Windows Store has now exceeded 50,000 app mark. According to that site, Windows Store has exactly 50,341 apps. Windows Store was opened last October and since then the numbers increased constantly every month to reach this level.
For a comparison, Apple AppStore has about 300,000 apps optimized for iPad with all leading app names available on the platform. Windows Store has a long way to catch up with Apple AppStore both in terms of app quality and quantity. Microsoft is working to improve developer experience of Windows 8/RT with upcoming update codenamed Blue which will include no.of improvements on WinRT.