My sense, without knowing the numbers but knowing quite a few Chapin families, is that there is a bit of attrition every year as girls discover this isn’t the right place for them. There are also a few new girls throughout… Really, it’s impossible to know as a parent or school who is going to thrive!
A decent number will leave for BS (or other schools, like Stuyvesant) after 8th and they will be “replaced”. My guess is that this varies from year to year though. It’s hard to know in 9th, for example, who was there in 8th vs who is new without having that conversation with the schools. And based on the families I know, none were certain they were leaving for 9th or 10th until they knew their options in the spring.
I believe Chapin has sibling preference (although I am not 100%).