I'd say it depends on your definition of "starts to lag". Is below 40 fps considered lagging? Below 20 fps (definitely noticable) or is anything not 60 fps considered lagging?
It also depends on what the entities are, you could probably have millions of points without lag (that's how 3D Tiles handles rendering point clouds. Those ones are static though). I linked to this blog post in your other thread:
You might also find this one useful:
Depending on what your application is doing, using something like model instancing or some 3D Tiles formats might be better options.