I can't even get sanctuary hills to go over 8.
I've got a beacon there, turned on and calling in new people.
And we've plenty of defense, food, water and excess beds.
Not entirely sure why no one will turn up.
I'm also not sure, but I think most of my population is made up of my companions, if they actually count towards the limit. (I think they do, because the airport registered as one, after I sent Danse to it).