I am a UX/UI designer and I have designed many mobile apps for iPhone and Android. Now I have to design a mobile app for Windows 8 but I don't know how many screen resolutions are available. i.e. for iPhone there are two resolutions (320x480 & 640x960).
If anybody knows then please let me know.
Microsoft supports a number of resolutions for Windows Phone 8: http://msdn.microsoft.com/en-us/library/windows/apps/jj206974(v=vs.105).aspx#BKMK_Supportedresolutions
I advice you to take a similar approach as you would with Android. This because Android uses a much wider variety of resolutions than Apple. These variety of resolutions leads to a (more) fluid design where elements don't have fixed widths of heights. This approach also makes your app future prove: who knows what resolutions Microsoft is going to support in the future.
Apple actually has more resolutions for the iPhone since iPhone5 (1136 x 640). The 640x960 is the retina version of the 320x480. The higher resolution displays everything a lot sharper but is not really handled as a different resolution the way you're used to.
