501st

The 501st most commonly refers to the 501st Legion, a fictional elite clone trooper unit in the Star Wars universe that served under Anakin Skywalker and later became Darth Vader's personal stormtrooper battalion.

The name was adopted by a real-world Star Wars costuming organization whose members build screen-accurate costumes and appear at conventions and charity events: The 501st showed up at the children's hospital.

As an ordinal number, 501st simply means the position after 500th.