how many files can a disk hold?

What is the limit on the number of files in a directory? On the number of subdirectories in a directory? How deep can the directory tree go?

I was stress-testing a new hard drive, by simply having bash write files to the drive, and the system crashed somewhere around 93 million files, after consuming about a quarter of the 2TB drive.

Obviously, no one would want 93 million files in a directory [pity Finder!] but it did get me wondering.