Parenting - performance Question

Is it good for performance to have under one objects parented other 100 objects and under them other 1000 objects? So one parent can have 10000+ objects in it.

Will it have impact on performance?

AFAIK, it makes no difference unless you’re calling methods like Transform.Find or GetComponentInChildren that have to traverse the hierarchy tree.

It depends what you have on those child gameobjects. Having hundreds or thousands of scene objects is not a problem, but if you are talking about hi res models than it is. If you are going to use methods to find something on all child objects that will be an impact on your performance.
If you are thinking about having one hundred thousand objects in your scene, you might want to rethink that concept. It is quite unusual.