Apple hasn't yet revealed how much of the 8GB of total storage is reserved for apps. As of now, no third party apps can run natively on the Apple Watch, they're actually installed on the iPhone and displayed on the watch. This does save on storage space but means they can't work if you're not paired with an iPhone. The apps Apple provides can be installed on the watch itself and many can offer most of their functionality when no iPhone connection is present. There is 2GB of storage reserved for personal data, such as music and 75MB is available for storing photos.