Aside from your question: prefer power-of-two in atlas texture sizes, so e.g. 2048, 4096, 8192. Using 8000 or 4000 as max resolution is not something we would recommend, unless you have specific reasons for that.
But the thing is, it said it cause some performance issue isn't it? It's better if it's 1 single png image isn't it?
Yes, having multiple atlas textures will produce additional draw calls, so having a single atlas texture would be preferrable in general. However, 8K is really large and having a single 8192x8192 texture might be problematic depending on your target hardware. Even if the hardware supports it, you should always perform measurements on the target device (using release builds) to see if a single texture is better or worse than having multiple smaller textures.
When having multiple atlas page textures, you can still improve the situation by having the draw order not switch between textures often, as described here:
spine-unity Runtime Documentation: Material Switching and Draw Calls