I read that the number of people you can send to Sanctuary is dependant on your charisma level. You get 10 slots and then 1 slot for each point you have in charisma.
I have 19 people in Sanctuary.
I have 7 points in charisma and no gear or buffs that are adding to charisma. By the logic of the advice I have read I should only be able to have 17, so that rule doesn't appear to be applying unless I'm missing something.