Laurence Tomei and Douglas Nelson say that, as mentioned, online undergraduate classes should have no more than 12 students. In person, on campus classes should be no larger than 18 students and hybrid models should be only 17 students. Undergrad classes should likewise have an upper limit of 18, while graduate classes should have no more than 14 students and doctoral classes should be just nine or fewer students, though Tomei advised more research around that doctoral level finding. – Forbes