Canvas Structure for Different UI Panels

I have a few general questions related to Canvas usage related, here are those:

1. Does it good practice to create different screens in Single Canvas? Like the main menu, game over, pause dialog etc... 2. If we change any Canvas element's value then all Canvas elements rendered again? whether they are enabled or disabled... 3. when to create multiple Canvas in the Scene? 4. I was getting Canvas update related big spikes in Unity Profiler after implementing this structure and Game HUD panel updating multiple times during gameplay - Please check below image so what is the cause?

Please share your knowledge regarding my above questions so Canvas and under it UI creation related concept become more clear :)