The ISS' SATCAT ID is 1998-067A. The CubeSats and other objects released from the space station are considered its sub-units, thus using different suffixes: 1998-067C, etc. The same principle applies to satellites which break up in orbit.
Apparently, the suffix might be up to 3 letters long, thus giving us 17576 sub-units per one launch.
What happens when that space gets exhausted? How are IDs assigned to the objects outside of that range?