I think mine came in around 20 - 21, but my daughters are already breaking through and she won't be 16 for a couple of weeks yet.
I had to have all 4 out at once due to crowding. Dad didn't want any damage to the money he'd already spent on braces.
I'm hoping, but not hopeful my daughter can keep hers just cause I remember that painful week quite well.
If the dentist says you can, I'd certainly keep them. Never know when they may be the last ones you've got in there.