what is the significance of Jon Snow being the 998th lord commander? it's not quite #1000, it's not even #999 (which is THE number that's significant for.. not being #1000). why even explicitly name him as being 998 in the books, if the number doesn't mean anything?
I think it shows that Jon’s ultimate narrative destiny is not at the Night’s Watch. His time there is an important part of his story (understatement!), but staying there is not for him.
Lord Commander #999 will probably be from the mutineers.
The numbering is significant for a character who will end up in the Night’s Watch as their endgame. They get the significant #1000th LC of the Night’s Watch numbering. Some think this will be Stannis; I tend to think Jaime.