How many 1024x1024 texture i can use in one scene for a 2D game ?
This is really only limited by memory, and that will all depend on what else you have loaded at the time, as well as whether you use compression on your textures. PVRTC compressed textures will get it down really far to where you can use a lot of textures assuming you don’t have huge audio files loaded, etc.
EDIT: One more note: while you can use lots of textures, you really don’t want to display objects that make use of more than a few of them because for ever material that is being used to draw an object, you incur another draw call. And the more draw calls you have, the bigger the hit on performance. So you want to keep your draw calls to as few as possible, and the way to do that is make sure as many objects in the scene as possible (at any one time) share materials.