Yah, really truly possible to have 15 or more, and to have one alter out for months or years and for the others to lose that much time.
Best support you can give her is just to believe her and accept all the parts of her that you meet and get to know. Really. That's all you need to do. Don't need to be her therapist or try to fix her.
The best gift anyone could give to me and mine would be just to let us be who we are and not get all freaked out by our switching.