I’d expect that you’d invite them once and then you could assign the one reference letter they uploaded to as many colleges as you want without them being spammed with multiple emails, right? Or do they get a different invitation link for every college you’re applying to?
As a teacher, I write one letter per student.
After Aug 1, I upload it onto the kid’s Common App, and answer the checklist questions on the student. I also submit a hard and uploaded copy to guidance for any non-Common App schools to which the student is applying.
And then I’m done. I don’t get any invitations from Common App-- perhaps because in my school the letters are due before application season begins. We had to get them in for this year’s seniors before school ended in June.