How to avoid the screen turning on for each notification?
I Googled around and am afraid this is the default setting for Android 7. People from all over the Internet are complaining about this. You can't simply turn it off.

However I found this trick working by entering Do Not Disturb mode. In Settings -> (Modes) -> DND there's a switch named Block visual disturbances. Turning it on and putting your phone into DND mode should work. As a side-effect, some other things are also changed in DND mode.