Don’t know the limits, but once a month one of us copies approx 40,000 directories, each with 2 512K files and a 1k file. so thats 120000 files. (from a server to a USB hard drive on a workstation) The 40,000 directories are selected from approx 100,000 in the parent.
using windows explorer it has to be done in chunks or it will sometimes fail. Using a DOS prompt it works with no problem.
When doing it “in chunks” I would generally select about 2000 folders at a time.