At my institution the salary stays pretty constant until enrollment reaches 20. Starting at 21, you are given your base rate plus 1/20th of your base rate. This continues to increase up to 29. Once your course enrollment hits 30, your are given an additional load (which means that you are paid as if you were teaching two classes). Not that many of our classes have enrollments of 30 or more (that is a heavy load for an online class--I know, since I am one who is teaching a class of 30 this quarter).