Today, Microsoft announced more details around its Project xCloud game streaming service. Most importantly, developers won't have to make any modifications to their games for them to be supported by xCloud.
That means that technically, Project xCloud supports the over 3,500 games that are playable on the Xbox One, even including the Backward Compatibility list. That means that Xbox and Xbox 360 games will work as well. Moreover, Microsoft said that there are over 1,900 games in development for the Xbox One, so that brings the total to well over 5,000 games. And when a game is updated on the Xbox Store, it's automatically updated for xCloud.
Of course, the key words to pay attention to in the blog post are "technical capability". Just because a game is technically able to stream doesn't mean that it will. Presumably, this will be left up to the developer.
Microsoft also made some improvements to the Xbox Development Kit, adding a new IsStreaming API, which will let the game know when it's streaming. This way, game developers can add features that are specific to streaming.
Obviously, there's a lot more information to come. Presumably, we'll hear more at E3 in June.