I’ve worked extensively over the years with students in all three settings. I’ve had small groups of every size and classes of up to about 50 students. Presently my classes are generally 5 to 10 students. I think that the reason my classes have gotten smaller is that students are busier than ever and have trouble fitting a class into their schedules. So in recent years I’ve worked even more often with students individually and in small groups of friends. Each setting has its advantages. In a group setting, students can hear the questions of other students and often benefit from this. The large classes are more economical and usually students are able to get more hours of practice in this setting. Individually, the advantage is focusing on the exact needs of the student, perhaps spending more time on math or reading, depending on their needs.
Generally, a student is a better candidate for a group if they are closer to the middle range of achievement (unless they can become part of a group where they are all at a similar level). Students at the highest level of scoring on the SAT, or at the lowest, can usually benefit more from individual training than as part of a group, which may go too fast or too slow for their needs.