"Bob" is a computerized system to help you manage your class.
Different teachers have different ways to appeal to and win the hearts and minds of their students. Some teachers do it through the example of their professionalism, sometimes augmented by humor and love, and the confidence that the teacher is able to instill into their students that he will be able to take their English to a higher level.
But in large classes there are students who are slipping through the cracks. As you implied, how can they "focus on the performance of every one of their students under these conditions" rather than just, what I'll call, "broadcast teaching"?
This past semester I was experimenting with Bob and have written about it before.  In a way, "Bob" is a computer system I'm using consisting of an Excel file and Visual Basic program. But it is more an extension or appendage of me as a teacher and many of the things I'm doing with my students. Technology isn't running my classroom but I am using technology to help me do what I need to do easier and better.
I must say it has helped me keep track of my students ten times better than before I began to use it. Of course, I'm a bit absent minded and always have trouble remembering things, people, names, etc.
For people who are interested in this sort of thing you need to know what to do after you gain the ability to closely monitor each student. For example, built into the system you would have some Diagnostic and Formative Assessments.
The next step is grouping the students or segmenting them. (Bob enabled me to develop the name cards I spoke of earlier.) Then you can apply what they call "Differentiating Instruction" to better meet the needs of the students and their motivational triggers in a more individual way.
I think most teachers teach to the middle level of the class. We don't teach to the bottom students but we don't teach to the top students either. We teach to the middle students. This is perhaps the normal effort of teachers. Teachers who make a greater effort may try to do something for students who are lagging. But there is also a significant number of students who are doing very well in our classes. If we don't pay attention to their needs they will be stuck at an academic ceiling, possible boredom and lost opportunity to develop. We need to offer them a degree of more challenging work so they can climb higher than the class average to which we are teaching.
By effective use of available technology all of this is possible to the degree that I think you could manage 500 students.