The first time I taught a 100-student course, I felt overwhelmed by how different it was from a 20-student class. Suddenly, I had five times as many names to remember. Five times as many papers to grade. Five times as many emails to answer. It was an adjustment, but with time I discovered new tricks for learning names, I set boundaries for myself on answering emails and I moved from a discussion-based class to more lectures and group activities. After experimenting for a few semesters, I finally felt comfortable.
Then I was assigned a 400-student class.