ACKS RAW is 14, and that's what I use for playable races using classes from the book, to the extent they make sense in the game world. ACKS also has rules for custom classes, and I cap those at 12.
Dimmermount extended ACKS for characters reaching level 18 (after?) attaining immortality, and I've exploded that out to 16 for undead (to the extent they can have class levels), 18 for physical immortals, and 20 for true immortals (demigods)
That's probably needlessly complicated as I very much doubt anyone sees level 9, much less 12, but the stick needed a carrot to dangle from the end.