You might be interested to take a look at the following thread. GameObject’s are very costly and the editor starts to surrender when using a few ten-thousands:
Related issue-tracker item:
I believe Unity’s entity component system is solving this issue, but it’s only in preview at the time of writing. As far as I remember, this talk mentions the “megacity” scene contains millions of entities.
Entity Component System (ECS) “Megacity” walkthrough - Unite LA 2018 Keynote