With Harry not in the white council any more I doubt it's an apprentice like we are thinking. But between the members of the paranet and the regular people who join under his banner during the battle of Chicago I wouldn't be surprised if Harry has a lot of people training with him to various degrees. I feel like this might be a "best man" twisted by fae /winter situation. Probably someone Harry hates. Or it's marcone
I'm pretty sure the "wizard's apprentice" tradition pre-dates the entire White Council.
During 12M -- and this story is set
during 12M -- I don't think Harry's going to have time to set up any sort of multi-student training. One apprentice seems like plenty!
Agreed that there is plenty of need for Harry's training, plenty of students who would benefit. One popular theory has the Paranet becoming a less-elitist / more-egalitarian replacement for the White Council (which seems doomed to fall).
Not Marcone, though: he has Thorny to train him, and Namshiel is the Denarian's wizard-nerd, so Marcone doesn't need Harry.