Ideal number of atlases

I'm currently developing a game using NGUI plugin.

I understand having least number of atlases per game(or scene) is ideal for both draw call performance and memory management.

According to Unity3D memory profiler, 2048x2048 RGBA32 is taking up about 32MB memory on Unity editor. I currently have about 4 atlases (2048x2048), which I think is taking up way too much memory and eventually throws OOM on iOS.

Is there an ideal number of atlases that I can have per game (or scene) ?

Also, I am noticing that atlases (materials to be exact) are not getting unloaded from a scene when the scene no longer exists. I have destroyed all references & gameobjects, yet it stays there on Memory profiler with a reference to 'ManagedStaticReferences'. What exactly is 'ManagedStaticReferences' and how can I get rid of it?

Any forms of advise would be appreciated.

Thanks!