Agent 99. Agent 23. Agent 91. Agent 86 (Max). Agent 13. And... Larabee? Did they deem him too stupid-looking to have a number or something?
It's probably an Aerith and Bob type of joke. Men in Black did a similar thing with the twin aliens "(Unpronouncable series of noises)" and "Bob".
Just a random note, in the MIB novelization, the Twins are "Bob and Gracie". Kind of amusing that they did a joke about an unpronouncable name in a movie instead of a novel, when it would have been easier to do it the other way around.
Only field agents get a number. Larabee is support staff.
A support staff member who's trained to kill with Post-Its?
He probably couldn't remember his number.
It's possible that he has a number, but it's just never mentioned, sort of like an inversion of 99 having a number but no given name.
It could simply be that some agents just prefer being referred to by their number by their fellow agents and others by their name, and Larabee's one of the latter. Max has a number as well, but that doesn't stop everyone from just calling him 'Max' most of the time. In the movie at least, the only times we see Larabee are when he's in a secure facility surrounded by people who know him, not out in the field where his identity would need to be more closely concealed and where he might conceivably use his number, so it could simply be that in such circumstances he prefers being referred to by his name and everyone complies.