Answered Max number of resources in a container.

That depends on the server environment. The listing of the resources is done with an AJAX request, and it's probably timing out before processing and returning that many records.

Try installing something like Collections, and converting the parent resource to a Collection container. That way its children are listed in a grid in the parent resource's Children tab upon opening the parent resource for editing, and that grid has paging, so you'll never have all that many resources being listed at one time.