Selecting checkboxes based on previous selections

You can see how my checkboxes are displayed in the photo below.

What i would like to do is that they need to pick either one or two from the subjects, and depending on how many they picked, only that amount of teachers could be selected below.

Before they don't pick a subject, a teacher can't be selected.

I hope all of these make sense and you guys can help me. Thanks in advance!