The max image size for a texture atlas is 2000 x 2000 pixels. After the atlas has reached its maximum size, a new atlas is created automatically.
You do not HAVE to create a new atlas yourself as Xcode will do it for you but I personally prefer to keep it below the max so I know what images are part of what atlas without having to look into it. It boils down to your own preference.
Source: Apple's About Texture Atlases.