Yoga classes have not always consisted of one teacher and a dozen or so students. Historically, all yoga classes were private classes. Until recent times, the science of yoga was preserved by the transfer of knowledge from a teacher directly to a single student, one-on-one.
Private classes can be an excellent way for new students of yoga to be better-equipped for their first group class or for their own private practice. It can also be a great way for intermediate and advanced students to advance their practice by utilizing their teacher's undivided time, attention and wisdom.