One-on-one tutoring from a human expert is an effective way for novices to overcome learning barriers in complex domains such as computer programming. But there are usually far fewer experts than learners. To enable a single expert to help more learners at once, we built Codeopticon, an interface that enables a programming tutor to monitor and chat with dozens of learners in real time. Each learner codes in a workspace that consists of an editor, compiler, and visual debugger. The tutor sees a real-time view of each learner's actions on a dashboard, with each learner's workspace summarized in a tile. At a glance, the tutor can see how learners are editing and debugging their code, and what errors they are encountering. The dashboard automatically reshuffles tiles so that the most active learners are always in the tutor's main field of view. When the tutor sees that a particular learner needs help, they can open an embedded chat window to start a one-on-one conversation. A user study showed that 8 first-time Codeopticon users successfully tutored anonymous learners from 54 countries in a naturalistic online setting. On average, in a 30-minute session, each tutor monitored 226 learners, started 12 conversations, exchanged 47 chats, and helped 2.4 learners.