Airline pilot here …
Not that I’m an expert on this topic, but in general it seems each airline retires a flight number that crashed, while other carriers continue to use it. The logic is that what’s become (in)famous is the phrase (e.g.) “TWA800”, not the number “800”, and so somebody else, say Northwest, is free to keep using 800 without giving off bad vibes that scare away customers.
The whole idea that numbers scare people is nuts, but that’s human nature.
Interestingly enough, 9/11 seems to be an exception. As best I can tell, people don’t much remember the three flight numbers other than 93, and they can’t recall which airlines’ airplanes ended up where. Clearly the airlines involved were trying like mad to make sure their names were dropped from the news coverage as soon as possible.