There's no definitive, publicly available data on the exact number of people who use online traffic schools each year. This information is spread across numerous private companies and isn't centrally tracked. Furthermore, the definition of "online traffic school" can be broad, encompassing various programs with different focuses (e.g., defensive driving, traffic violation courses).
While precise numbers are unavailable, it's safe to say that millions of people use online traffic schools annually in the United States alone, given the high volume of traffic violations and the popularity of online learning. The number would vary considerably by country.