There is about 16k (sixteen thousand) different item types in the game. If I want to create decent competitor for Jita 4-4, will be nice to have all of them on this Jita-competitor market.
So I propose not 64 per level, but at least
16000/3 ~ 5.3k per toon
5.3 / 5 ~ 1,1k per skill level
lets say 1024 will look good enough
as i know, there is no explanation for existing order limit
the only one i assume - order limit exists by server performance reasons
but, if there is only performance limitation I could imply any logic for order limit, e.g. “one account (three characters) should be able to cover whole market”